The Romulan Republic wrote: ↑2019-03-31 11:19pmAlthough its not spelled out in detail on-screen, its not hard to figure out a plausible sequence of events:
To question 1): How much territory the FO controls is never stated, but it clearly includes control of some planets. They're definitely not an insurgent force- they very clearly are armed like and fight like a conventional military power. No, they don't control the whole galaxy, but they pretty clearly don't need to control the whole galaxy to build a planet killer. The Empire built two death stars largely in secret, meaning it was a small enough fraction of their resources that they could pretty much hide the expenditure.
If it was up to me, I'd add something about Starkiller initially being an Imperial weapons project that the FO took over, as well. But that's just my personal theory.
As Jub mentioned, it's easier to buy the Empire pulling it off because they basically ruled the entire galaxy with only a partisan resistance and a few independent fiefdoms operating outside their control. The sequels, on the other hand, are incredibly vague about what the First Order actually is, so even answers to simple questions like "how many systems do they control" can change on a moment-to-moment basis.
That said, I would have personally been all for having all the super-advanced First Order tech actually be old Imperial projects they salvaged. It would distinguish them from the Empire by establishing them as a threat, but for different reasons. While the Empire utilizes horrific weapons very deliberately in order to maximize the terror created as part of a campaign to cow the rebellious population into submission (see destroying Alderaan instead of Dantooine because the latter is too remote), the First Order uses them on fleeting whims because, having effectively just been given this technology on a silver platter, they have no real respect for the power of what they possess. They'd be the Star Wars equivalent of those rogue states or terrorist groups from Tom Clancy-type stories who get their hands on nukes that fell through the cracks after the break-up of the Soviet Union, and then start nuking everything they don't like just because they can.
Instead, the FO pulls superweapons out of their butts, effortlessly decapitates the only other even remotely established superpower in the setting, and immediately jump to the level of overwhelming military force that no one is apparently willing or able stand against. And for the cynics among us, a major reason for this happening is because making it about heroic partisans versus evil empire makes it more like the OT.
Won't comment about questions 2 and 3, because the New Republic was so poorly established in the movies it's almost comical. When I originally saw TFA, I thought Starkiller Base blew up Coruscant because they don't even bother telling us what the names of the planets are, so I inferred that destruction of the Republic capital must have meant destruction of Coruscant. Hell, I don't think they mention it in TLJ, either, so I'd probably still think it was Coruscant that was destroyed if I didn't come here. At that level of world building, asking why the New Republic didn't do more to contain the First Order is like asking why Narnia wasn't doing more to stop Hitler during WWII.