Straha wrote:During the second Punic War Rome raised 26 legions at its height (roughly 500,000 men, give or take. Unless I'm off by some, Thanas?), projected that force across three theaters (Italy, Spain and Sicily) and maintained a large Naval force as well, all while being denied access to a good portion of its potential Manpower because of cities which had joined with Hannibal and having lost multiple legions already to Hannibal.
Actually, the manipular legion of the time had a nominal strenght of 4500 men (300 horsemen, 1200 light infantry, 2400 heavy infantry equipped with sword and javelins and 600 veteran heavy infantry equipped with sword and spear). 26 legions would have been 117000 soldiers divided into 7800 horsemen, 31200 light infantry, 62400 heavy infantry and 15600 veteran heavy infantry. All of this WITHOUT counting the 'artillery', as the Romans started using ballistas in that period and sometimes used it as field artillery (especially the lighter version called scorpio).
Now, for the confront, I don't know a lot about the Qin armies of the time, but I have the impression that their main strenght was in the composite bows and crossbows of their infantry. If I'm right, the Qin wouldn't stand a chance against the Romans: the ballistas and scorpios would outrange the Chinese archers, and while Roman cavalry of the time was not exactly an elite their infantry would slaughter the Chinese armies in the melee, thanks to a combination of better melee weaponry, better morale (volunteers against conscripts), better training and a nice barrage of killing and disrupting javelins on the Qin heads five seconds the beginning of the melee. Superior combined arms won the day.
Against the Punic armies, the Qin would have similar problems: while the inhabitants of proper Carthage were for most part sailors, they had a varied and strong army composed of Lybian heavy infantry normally fighting in phalanx, lighter infantry from Lybia and Hiberia, medium Gaulish and Sardinian infantry, the famous Numidian light cavalry, Iberian and Gaulish heavy cavalry, the infamous war elephants and, as the elite, a phalanx of Carthagians devoted to win or die (similar to some Greek elite corps like the Theban Sacred Bands, only larger) and capable to fight the Roman armies and win, if well led (like at Tunis). The equal and opposite of the Roman legions of the time, where only the commander would win the day. Against such an army, the Qin could use their archers well, but would have a problem against the war elephants (at the time the Romans were the only force capable to defeat them, and only thanks to their insane barrages of javelins and a series of previous bad experiences).
Against the Marian legions, it would be a one-sided slaughter: thanks to the conquest of Lybia and Numidia, the quality of the Roman cavalry is now improved, the Roman infantry has better javelins and more training (as they're now professionals and not the average Roman citizen who trains once in a while during peace), and the field artillery is made by more numerous and better ballistas and scorpios (the Romans continued to work on always better versions of the weapon for a long time).
Now, the marauding hordes of Gauls. Or, better, the disciplined hordes of Gauls: given a capable commander, they proved quite a force to be reckoned (asks Caesar about the Siege of Gergovia), thanks both to numbers, better discipline most people gave them credit for and the general understimation they were victims of. The Gauls could be just a nuisance or a major problem, it just depends if they find a capable commander and he manage to be obeyed.
Against the Germans, instead, the Qin would win easily: at least until the Romans started to use and train them as soldiers, the Germans were just hordes of ferocious men on foots with swords and axes plus some berserker. They may give the Qin a rule for their money and even beat them if they manage to pull an ambush in favorable position, but in a normal field battle the archers would just slaughter great numbers of them before the melee with a fresh and better equipped and trained infantry and the contious attacks from light cavalry and war chariots and charges of heavy cavalry.
I noted the OP missed the two last major forces of the time, Parthians and the Diadochi. I'll cover them at the best of my knowledge.
First the Diadochi and their armies. Against them, the Qin would win thanks to combined arms: while Alexander's army could use complex tactics and the combined arms of the Macedon Phalanx, heavy (and well trained) cavalry, hoplites phalanxes, light infantry armed with bows and javelins and, after the conquest of Persia, the Persian forces of archers, war chariots and war elephants, the Diadochi ended to use only the Phalanx and let the other components of their armies degrade in quality, when they didn't dismantled them. Against archers it would be a one sided slaughter, and the Qin archers would have the right to complain for the boring job. And that's without calling in cause the rest of the army.
Now, the last force, the Parthians. An organized force based on fast moving mounted archers and heavy professional cavalry (sort of knight predecessors and elite of the army), with not very good infantry. For what I know, the Qin army was composed by the mentioned archers, a passably trained and relatively good equipped conscript infantry, war chariots and cavalry, both nomadic mounted archers and heavy cavalry. The Qin army should have the advantage: as the standard Parthian tactic was using the mounted archers to softer the enemy until the heavy cavalry would see the good time for a crushing charge, the prestigious Qin war chariots and the nomads would be a major problem for the Parthian mounted cavalry and reverse the scenario as the relatively faster nomads soften the Parthian heavy cavalry to the point the Qin heavy cavalry would be capable of fighting on par the Parthian knights or the Qin infantry would actually CRUSH them, as during the charge the weakened Parthian knights would have to endure the Qin archers all the way and then fight an infantry trained exactly to fight cavalry (both nomads, if they managed to catch them, and Chinese).