40K question

[Sarevok]

I am mainly a Dawn of War game player, I have a few of the 4th and 5th edition rulebooks and codexes as well because I liked the artwork. But I dont play the board game and havent read the novels yet so forgive my ignorance here.

My question is the contrast between ground and space aspects of the setting. Gauging by 40K analysis threads here 40K space weapons are almost as powerful as Star Wars (latest ICS figures). So they have teraton yield starship weapons and factions like IoM has millions of warships. And nobody has planetary shielding technology. Barring super rare vogue mentions almost no planet has anything resembling an energy shield. There is nothing preventing a warship slagging a planet with the multi AU ranged weapons that are so common on 40K warships. So how come the universe is entirely centered around ground combat when one point defense gun on a freighter can vape a continent ?

[Norade]

Usually a conquest on the ground has a goal of more than just kill everything and move on. Chaos wants cultists and to capture equipment from fallen codex marines, the Orks love a good fight, the Eldar won't put a planet to the flame lest they fuck with the warp again, Dark Eldar want captives, the Nids do nuke planets as quickly as they can, the Tau can't reach anybody, the Guard and Marines often want to hold something or recapture something important, and the Necrons usually just pop out of the ground and force you to fight or leave.

[Sarevok]

Usually a conquest on the ground has a goal of more than just kill everything and move on. Chaos wants cultists and to capture equipment from fallen codex marines, the Orks love a good fight, the Eldar won't put a planet to the flame lest they fuck with the warp again, Dark Eldar want captives, the Nids do nuke planets as quickly as they can, the Tau can't reach anybody, the Guard and Marines often want to hold something or recapture something important, and the Necrons usually just pop out of the ground and force you to fight or leave.

Well every faction would benefit from orbital bombardment even if they wanted to take the planet. They can destroy PDF and Imperiul Guards formations with impunity from space and then waltz over the battered remmnants. This is 40K so it is not like anyone cares about collateral damage from nuking a IG garrison with space based weapons. Even mighty space marines are a cakewalk when every concentration space marine defenders on a planet just goes up in mushroom clouds.

[Norade]

Usually a conquest on the ground has a goal of more than just kill everything and move on. Chaos wants cultists and to capture equipment from fallen codex marines, the Orks love a good fight, the Eldar won't put a planet to the flame lest they fuck with the warp again, Dark Eldar want captives, the Nids do nuke planets as quickly as they can, the Tau can't reach anybody, the Guard and Marines often want to hold something or recapture something important, and the Necrons usually just pop out of the ground and force you to fight or leave.

Well every faction would benefit from orbital bombardment even if they wanted to take the planet. They can destroy PDF and Imperiul Guards formations with impunity from space and then waltz over the battered remmnants. This is 40K so it is not like anyone cares about collateral damage from nuking a IG garrison with space based weapons. Even mighty space marines are a cakewalk when every concentration space marine defenders on a planet just goes up in mushroom clouds.

True, but when that firepower needs to go towards killing the ship the Marines or Guard just arrived on that leaves time for a ground battle. There are also planetary defense guns that might make a ship getting close enough to fire suicide.

[Feil]

Planetary shields do exist, although single-city shields are much more common. The Eldar get around the problem of defense by just not having planets.

There are some attempts to justify space support not implying instant victory for any ground force with more than, say, 1% the fighting strength of their enemy. Some make considerably more sense than others.

On the sensible side are giant fuck-off laser cannons on the planet to burn down projectiles and provide counter-battery fire against space-based lasers, defense fleets filling the same role, isolated worlds which simply lack any measure of space-based weapons, and all the myriad combat scenarios in which 'nuke it from orbit' doesn't make tactical sense.

On the less sensible side are 40k warships somehow not being able to hit 'stationary'* targets from a scant thousand kilometers with weapons which can accurately target 50-meter-wide, actively evading space ships from a hundred times that distance; 40k spaceships not being able to see through clouds; and my personal favorite, that orbital bombardment will only create more places for the enemy defenders to hide.

*read: moving and accelerating in such a way that anybody with a computer can perfectly forecast their future location based off their present condition for thousands of years into the future.

In the end, however, it boils down to something which doesn't have anything to do with the internal consistency of the universe. Warhammer 40,000 is, for the most part, World War II in Space, with occasional intrusions of the Crusades in Space, Vietnam in Space, the Boer War in Space, and the Great War in Space. None of these wars were characterized by the ability to reduce any particular square mile of territory to radioactive rubble at the push of a button, so 40k files orbital bombardment away along with all the reasons why melee weapons and machine guns don't mix, cuts up the file with a chainsaw sword, and then nukes the scraps from orbit.

[KlavoHunter]

Yeah, I tried reading a 40k book that ended in "And then the Imperial fleet bombarded the Chaos titans into molten scrap.". Anticlimactic as HELL...

[Black Admiral]

As has been said by others, there are myriad reasons for orbital bombardment not being used; wanting to take a given location intact is the usual one - orbital bombardment isn't exactly subtle - but there are others, some making more sense than others (heavy enough surface to space defences that it's impractical, not having a target to shoot at, etc.).

Of course there are times when there's just no explanation (IA8, for instance - I can't recall any explanation being provided for why the Raven Guard (one of the more pragmatic Chapters) didn't just flatten Buzzgob's Gargant yards from orbit).

[The Grim Squeaker]

As has been said by others, there are myriad reasons for orbital bombardment not being used; wanting to take a given location intact is the usual one - orbital bombardment isn't exactly subtle - but there are others, some making more sense than others (heavy enough surface to space defences that it's impractical, not having a target to shoot at, etc.).

Of course there are times when there's just no explanation (IA8, for instance - I can't recall any explanation being provided for why the Raven Guard (one of the more pragmatic Chapters) didn't just flatten Buzzgob's Gargant yards from orbit).

40K actually frequently bothers to explain this, even in many short stories (unlike SW).

City wide defense shields seems extremely common and ludicrously durable, to the point of the area surrounded by said shields being the only parts of the planet left (on one occasion), in addition to the oft noted examples of them being able to ignore orbital bombardment while firing back with heavy set artillery. (I can think of at least 2-3 short stories offhand that have a small team of SM's drop in a suicide raid to drop the shield or disable the AA cannon to let the invasion land/bombard/attack, etc').

[open_sketchbook]

One of my favourites was in Seige of Vraks, where the defenders had an orbital gun battery powerful enough to make any attempt to attack with a fleet basically sucide. The Imperium kept their fleet on the opposite side of the planet from the guns, landing troops to grind down the defenders. If my memory served, it was decided that the millions of casualties required to take the planet would be the lesser cost, compared to risking Imperial ships.

I think that how valuable ships are compared to ground assets is probably a big part of it. Ships in gravity wells are quite fucked if other ships move in; when the Imperial fleet got caught around a gas giant in the Third Armegeddon War the Ork fleet ate them alive, and orbiting a planet in Battlefleet Gothic is usually a death sentence. It may be the risk of sticking around to bombard ground targets is considered tactically unwise; better to stay farther out where they can react to attackers. If hovering about in orbit gets your ship blowed up, the whole thing is a wash anyway as the newcomers will then have orbital superiority and can wax your force.

[Juubi Karakuchi]

Dan Abnett's 'The Sabbat Worlds Crusade' describes at least one instance where orbital bombardment is used to soften up a planet before invasion. If I remember correctly it was Balhaut, which fits if the Imperium tends to do so when a planet is particularly heavily defended, but must nonetheless be reclaimed.

[Serafina]

Opposing fleets often play a big part in it as well. If your enemy has a fleet that can threaten yours, the using your fleet for orbial bombardment will be difficult. Maneuvering plays a big role in 40K space combat, and if you move your fleet into position for a proper orbital bombardment (which frequently involves lower orbit), then you might put yourself at a disadvantage. The enemy can swoop in and take out a chunk of your fleet. And if that's enough to tip the balance of power, you just lost your fleet AND the planet.

That gives us a couple of szenarios as far as orbital bombardment goes:
-A fleet is attacking a planet without defenses (against starships via strong weapons or shields) or a defense fleet. They don't care much about the planet - not for it's strategic position, resources or for prestige. The usual result is that the planet will fall quickly, even if it has a seizable army to defend itself (rare if it doesn't have any space defenses). A example for this is Tanith, which was completely destroyed by a small chaos fleeet in a single night.
-A fleet is attacking a planet without defenses or a defense fleet (or has a big numerical advantage), but does care for the planet itself for some reason. This will put the attacker at a serious advantage. The main advantage that we see is high troop mobility - they can land nearly everywhere they want, sometimes even right on top of the enemy positions. The second advantage is orbital fire support, especially lance strikes against specific targets (fortifications or war machines). We often see this in Space Marine campaigns.
-A fleet is attacking a planet with a sizeable defense, but no defense fleet. The result is typically that they have to land their troops somewhere where the defenses can't hit them and fight a prolonged land war. Once you've taken down the defenses, you might as well capture the planet rather than burning it down (tough sometimes that's still necessary, e.g. Chaos). The Siege of Vraks is a good example.
-A fleet is attacking a planet without defenses, but a sizable defense fleet. The attackers will have to draw the defending fleet away and land their own troops. That means that both fleets wil stay busy - they have to be alert in order to keep a tactial advantage in space, and they have to keep the enemy fleet away from their ground troops. Occasionally, one side might risk to get near the planet, to land further troops or give fire support against a strong target. We often see this respresented in the game, where orbital support can only be used once or for a short number of rounds - the spaceship won't stay long and is just flying by.
-A fleet is attacking a planet who has both strong defenses and a sizable defense fleet. This gives us a mix of the last two szenarios - a very long, drawn out land campaign with minimal orbital support for either side. However, since the defending fleet would decide the land battle if they could win, those attacks are only carried out with quite a large fleet on the attackers site, so the defense are needed later on to prevent the attacking fleet from doing anything.

So that's it. Only one of the szenarios above allows free orbial bombardment, and only one other allows it to be used whenever necessary. Either opposing fleets of powerful defenses prevent starships from freely attacking ground targets.


Now, what defenses are there?
First, we have shields. Those are mostly used in large, static, defended areas - large fortresses or cities (or both combined). Some are only wide umbrella shields, so they don't do much against artillery, at least if it get's close enough to shoot under the shield. Some encase the entire city/fortress, preventing artilleryfire as well.
There are no large, mobile area shields (as far as i know), but psionics can actually put up storms powerfull enough to make orbital bombardment even more imprecise than usual. War machines often have shields that can stop the weapons of spaceships, tough sustained fire will crack those easily (but as mentioned above, that's not always possible).
There are a lot of weapons that can hit and damage spaceships even in high orbit. Those weapons range from very large, stationary defense lasers that put out more power than the lance weapons on battleships over mobile missile launchers that work similar to the torpedoes on spaceships to ground-based space fighters. Of course, there are also plenty of orbital weapon platforms, orbital fortresses, system defense ships etc.

[Black Admiral]

One of my favourites was in Seige of Vraks, where the defenders had an orbital gun battery powerful enough to make any attempt to attack with a fleet basically sucide. The Imperium kept their fleet on the opposite side of the planet from the guns, landing troops to grind down the defenders. If my memory served, it was decided that the millions of casualties required to take the planet would be the lesser cost, compared to risking Imperial ships.

Except that that explanation only "works" because everyone involved had the competence and tactical flexibility of a lemming. It's never actually explained why nothing can be done about the defence laser batteries on Vraks (such as, say, just using Astartes forces to knock out the planetary defence batteries, or at least enough of them to give a fleet room to start reducing the rest). Usually there's at least an explanation as to why that isn't practical, or it's what's actually done (see Dark Creed, with the Word Bearers doing exactly as described against a world vastly more heavily defended than Vraks), but no such explanation's given for Vraks. And don't even get me started on the abuse heaped on the Dark Angels (half their fucking Chapter present, and they get their nads handed to them by an Alpha Legion force they completely outclass - out, and apparently Caliban is now intact and the DAs' main recruiting world)).

Then, Forgeworld has some very odd ideas about Astartes (see; the wildly out of character Raptors (IA3) & Raven Guard (IA8)) generally.

[Simon_Jester]

Except that that explanation only "works" because everyone involved had the competence and tactical flexibility of a lemming. It's never actually explained why nothing can be done about the defence laser batteries on Vraks (such as, say, just using Astartes forces to knock out the planetary defence batteries, or at least enough of them to give a fleet room to start reducing the rest).

Were there enough Marines available to do that? If we're talking about positions defended well enough to burn up thousands of Guard regiments during the campaign, we're talking about positions defended well enough that it could take a lot of Marines to crack them.

The Imperium generally doesn't send in the Astartes in situations where they know a bunch of them are going to die, not in the fluff, not unless it's truly desperate. They can't afford to. It is entirely in keeping with the general patterns of 40k behavior for the Guard to be sent in to do they dying en masse in situations where it's not deemed worth risking a Marine force.

[Black Admiral]

Except that that explanation only "works" because everyone involved had the competence and tactical flexibility of a lemming. It's never actually explained why nothing can be done about the defence laser batteries on Vraks (such as, say, just using Astartes forces to knock out the planetary defence batteries, or at least enough of them to give a fleet room to start reducing the rest).

Were there enough Marines available to do that? If we're talking about positions defended well enough to burn up thousands of Guard regiments during the campaign, we're talking about positions defended well enough that it could take a lot of Marines to crack them.

The ground level defences of the laser silos aren't ever detailed, or even mentioned if they have any - it's simply taken (within the text) as a given that they can't be knocked out without any explanation as to why. And, as I noted, there is an example of just such an assault against planetary defence batteries being executed (by a Word Bearers force (34th Host, IIRC ~1,000-1,500 strong c. Dark Creed), admittedly, but it's hardly beyond loyalist Astartes) against a far harder target than Vraks (Boros Prime, which has a population of 10 billion (5 billion of which are serving or have served in the PDF or Boros regiments of the Imperial Guard), as against Vraks' 8 million (hells, one city on Boros Prime - Sirenus Principal - has more serving soldiers (15 million from a pop. of 80 million) than Vraks' whole population), even leaving aside the White Consuls and the superior equipment of the active Boros PDF & Guard).

There's not really anything preventing a loyalist Astartes force from executing a similar attack (aside from that doing so would prevent Forgeworld from piling on the WW1 cliches and stripping the Death Korps of Krieg of anything interesting in the process).

Also, I'm not sure where the "thousands of regiments" figure is coming from. The Krieg 88th Siege Army's order of battle at the start of the campaign was;
1st Line Korps;
3rd Krieg Siege Regiment
5th Krieg Siege Regiment
15th Krieg Siege Regiment
19th Krieg Siege Regiment

12th Line Korps;
143rd Krieg Siege Regiment
149th Krieg Siege Regiment
150th Krieg Siege Regiment
158th Krieg Siege Regiment

30th Line Korps;
261st Krieg Siege Regiment
262nd Krieg Siege Regiment
263rd Krieg Siege Regiment
269th Krieg Siege Regiment

34th Line Korps;
291st Krieg Siege Regiment
308th Krieg Siege Regiment
309th Krieg Siege Regiment
310th Krieg Siege Regiment

8th Assault Korps;
7th Krieg Tank Regiment
11th Krieg Tank Regiment
14th Krieg Tank Regiment
179th Krieg Siege Regiment
231st Siege Artillery Regiment

11th Assault Korps;
61st Krieg Tank Regiment
66th Krieg Tank Regiment
101st Krieg Siege Regiment
497th Siege Artillery Regiment

19th Bombardment Korps;
3rd Siege Artillery Regiment
4th Siege Artillery Regiment
8th Siege Artillery Regiment

21st Bombardment Korps;
19th Siege Artillery Regiment
22nd Siege Artillery Regiment
23rd Siege Artillery Regiment

Independent artillery companies;
4th Artillery Company
6th Artillery Company
8th Artillery Company
27th Artillery Company
31st Artillery Company
33rd Artillery Company
224th Artillery Company
226th Artillery Company
227th Artillery Company
230th Artillery Company
61st Heavy Mortar Company
67th Heavy Mortar Company
70th Heavy Mortar Company
71st Heavy Mortar Company

They were reinforced by other units over the course of the campaign, but I can't find any specific listing of such.

[Elheru Aran]

Of course, if you want to go out-of-universe, the main reasons are:

1.) Games Workshop makes a shitload more money off the tabletop game than it does Battlefleet Gothic. Consequently, the tabletop version of the universe gets much more press, so to speak.

2.) While a good writer could do a combined fleet bombardment with ground assault, GW wants more RAAAH FOR THE EMPEROR/WAAAAAGH! hand-to-hand with all the guts flying around and whatnot. It's all about selling the little plastic men for $10 a pop, don't you know?

Returning to the in-universe... the way I see it, the mix of ungodly powerful area-defense shields and ungodly powerful ship firepower means that either you're going to end up with a fortified and reasonably intact island in the middle of a uncrossable no-man's land, or you're going to blow off half the planet to destroy the city. Neither are particularly desirable solutions, given that planets are fairly valuable real estate in 40K, unlike in Star Wars where they've got millions. So this is why they have all the ground actions, and that's another reason why no orbital bombardment-- I know I wouldn't want to trust a servitor-gunner's aim within more than a few km's from my body when it's shooting from orbit with multi-megaton weaponry!

[Simon_Jester]

Also, I'm not sure where the "thousands of regiments" figure is coming from. The Krieg 88th Siege Army's order of battle at the start of the campaign was;

[snip]

Yes. Look at the casualty numbers; each of those regiments sustained (some ungodly number)*100% casualties during the operation. In effect, they were throwing hundreds or thousands of regiments into that meatgrinder; they just kept plucking the regimental standards off the battlefield where the last wave of regiments died and assigning them to the next wave of recruits. Thus formally keeping the same order of battle, while suffering the total constructive loss of many many regiments for practical purposes.

[Black Admiral]

Also, I'm not sure where the "thousands of regiments" figure is coming from. The Krieg 88th Siege Army's order of battle at the start of the campaign was;

[snip]

Yes. Look at the casualty numbers; each of those regiments sustained (some ungodly number)*100% casualties during the operation. In effect, they were throwing hundreds or thousands of regiments into that meatgrinder; they just kept plucking the regimental standards off the battlefield where the last wave of regiments died and assigning them to the next wave of recruits. Thus formally keeping the same order of battle, while suffering the total constructive loss of many many regiments for practical purposes.

Oh, yes. Hadn't thought of it in that sense; my bad, sorry.

Still, I'm not convinced that the fighting for Vraks had to take anything like as long or expend nearly so many lives, and only did so (to absolutely no success) because everyone involved had the competence & tactical flexibility of a lemming. Look at the only initial plan to use any Astartes forces available - hurl them straight into the teeth of the strongest defences without even making a token effort to reduce them. It's no wonder no Chapter Master'd have agreed to the plan.

[Vendetta]

Orbital bombardment is only usable in game in the Epic ruleset. Epic rules are supposed to represent entire weeks long campaigns between massive forces (an "assault" in Epic is supposed to essentially represent an entire 40k battle in a couple of dice rolls).

The restrictions on it's use can give us some idea about the utility of orbital attack in battles - you have to specify a turn before the battle begins when your bombardment will take place, indicating that it takes some amount of time and planning for an orbiting ship to get into position to actually perform the attack, so it can't be used as a reactive measure (remember that your 40k battle is simply a couple of dice rolls within that turn).

For larger bombardments you also have to specify where the bombardment is going to hit, and the template is suitably massive that it would, in essence, wipe out everything on the table of a 40k battle all at once (and they're macroweapons, which means that ordinary units just don't get saves at all). So again, it takes some setup time for the big hits.

Precision strikes can only be called against superheavy targets, so we know that targeting of orbital attacks is sufficiently woolly that anything smaller than a Baneblade can effectively not be targeted as an individual item.

[andrewgpaul]

Actually, the Daemonhunters, Withc Hunters and Space Marines all get an orbital barrage as an army option. It's a bit weedy, though. The Imperial Guard can also take an Officer of the Fleet in regimental command squads, who can coordinate with naval aircraft squadrons and lance batteries onboard cruisers to disrupt enemy supply lines and interdict reinforcements to the battle being played our on the tabletop.

[Lord Revan]

Actually, the Daemonhunters, Withc Hunters and Space Marines all get an orbital barrage as an army option. It's a bit weedy, though.

Well besides game balance (you wouldn't want to an army a weapon with an effect "use this for instant victory"), 40k actually do care about colledar damage, just differently then we do, for example an imperial guardsman is fairly easy and cheap to train and arm you loosing them friendly fire isn't so bad. Space marines and/or inquisitors take alot of effort to train and loosing one to enemy fire is a big deal, loosing one to friendly fire is unacceptble (and heresy if intentional). Any orbital support fired near those would probably be set to low power.

[Connor MacLeod]

Theatre shields are one reason.


Supposedly accuracy is one reason limiting accurate bombardment. Even the Imperium as brutal as they are do not want to totally fuck up planets and would like (in theory) to limit the damage, which rules out indiscriminate bombardment in some/most cases, and may very well rule out some of the bigger guns. I suppose it could be the only weapons that can accurately bombard are the point defense ones (one reason perhaps why they have to get so close, although in practice even the anti ship ground weapons are limited by range. Out of Universe its just a game mechanic but its never been adequately explained in universe other than "fairy dust".) But that doesn't cover all explanations either.

The more likely reason for the ground combat is probably one of politics, cultural and general "cuz I want to" human type factors. As I said before, the Imperium's bureacracy and politicians don't always just want to fuck up a planet for the sake of fucking up a planet (economic, industrial, or political reasons.) and orbital bombardment's collateral damage may be deemed undesirable. EG "Take that huge city for me and I don't care how many people die!". Then there's culture. A fair bit of the military in the Imperium is very tradition bound, and sometimes those traditions may be silly (fighting style, uniforms, etc.) This covers stuff largely like "honor" or such, especially for officers and their prejudices. Or they are indoctrinated that way. Or they are just stupid, lazy, or political/ambitious. OR whatever. "because I want to" also is in this category for much the same reason.

However, some guard officers (or Navy officers, or Space Marine commanders) ARE known for "bombarding the fuck out of stuff" rather than risking direct ground combat if the target is not deemed important and/or there is risk of taint.

Amusingly the recent Horus HEresy novel "Prospero Burns" by Abnett has a bit where a 40K starship is using (IIRC) heavy batteries firing megawatt laser beams to disable a target. So go figure.

[Simon_Jester]

Actually, the Daemonhunters, Withc Hunters and Space Marines all get an orbital barrage as an army option. It's a bit weedy, though. The Imperial Guard can also take an Officer of the Fleet in regimental command squads, who can coordinate with naval aircraft squadrons and lance batteries onboard cruisers to disrupt enemy supply lines and interdict reinforcements to the battle being played our on the tabletop.

The way that the Officer of the Fleet's fluff is described...

"It is standard procedure to attach an officer of the fleet as a liaison to commanders on the ground. Officers of the fleet are proud, aloof, and stern men. They coordinate with Imperial Navy bomber wings and even the lance batteries onboard warships. While the full might of the Imperial Fleet cannot be called upon, the available firepower is enough to disrupt the enemy's supply lines, forcing their reserves to take shelter or face destruction from above. Such actions delay enemy reinforcements from entering the fray, allowing the Imperial Guard to annihilate the foe one unit at a time."

This implies that orbital bombardment is mostly used as a strategic weapon: the Imperial Navy's bombardment is directed off-screen, as it were, and often simply with the objective of deterring the enemy from concentrating in inconvenient locations.

[mr friendly guy]

There are some attempts to justify space support not implying instant victory for any ground force with more than, say, 1% the fighting strength of their enemy. Some make considerably more sense than others.

On the sensible side are giant fuck-off laser cannons on the planet to burn down projectiles and provide counter-battery fire against space-based lasers, defense fleets filling the same role, isolated worlds which simply lack any measure of space-based weapons, and all the myriad combat scenarios in which 'nuke it from orbit' doesn't make tactical sense.

One of my favourites was in Seige of Vraks, where the defenders had an orbital gun battery powerful enough to make any attempt to attack with a fleet basically sucide. The Imperium kept their fleet on the opposite side of the planet from the guns, landing troops to grind down the defenders. If my memory served, it was decided that the millions of casualties required to take the planet would be the lesser cost, compared to risking Imperial ships.

I think that how valuable ships are compared to ground assets is probably a big part of it. Ships in gravity wells are quite fucked if other ships move in; when the Imperial fleet got caught around a gas giant in the Third Armegeddon War the Ork fleet ate them alive, and orbiting a planet in Battlefleet Gothic is usually a death sentence. It may be the risk of sticking around to bombard ground targets is considered tactically unwise; better to stay farther out where they can react to attackers. If hovering about in orbit gets your ship blowed up, the whole thing is a wash anyway as the newcomers will then have orbital superiority and can wax your force.

Forgive my ignorance, but if these planet anti ship defenses are good enough to repel capital ships, how the hell do invaders even land ground troops?

[Srelex]

Forgive my ignorance, but if these planet anti ship defenses are good enough to repel capital ships, how the hell do invaders even land ground troops?

It depends. Tyranids simply swamp them with spore pods, Tau and Eldar can sneak by, I believe, Orks simply spam Roks, and so on. Often, it may be a case of the huge warships being more of a priority against smaller, harder-to-hit transports.

[mr friendly guy]

Often, it may be a case of the huge warships being more of a priority against smaller, harder-to-hit transports.

If thats the case, the warships should still have time to fire WMDs against the defenders, since it must stay alive long enough for the landing party to reach the surface, or else the defenders will just focus their weapons on the landing party while its still in the air.