True, the spinoffs do show a lot of that, but that's getting away from the original Episode IV-VI argument.

Even Episode I-III already showed a bit more of what lead to the downfall of the republic.

When you look at real-life Germany or Austria leading up to the Nazis you had quite similar situations: instable democracies in permanent crisis mode that were gridlocked and finally replaced by fascist regimes.

What's the big difference though between the lead-up to the nazis and to the empire is the government that came before.

Both Germany and Austria were (financially) destroyed after WW1. Democracy was very new in both countries, only 15 years at the time when the nazis/austrofascists took over. The democratic systems of these two countries were full of holes and weaknesses. Political extremism was very strong while the political center was weak.

The extremist parties has their own paramilitary forces that ended up being stronger than the official forces of the state, and when the great depression hit and destroyed the livelihood of most people, protest votes pushed the fascists into power.

So the main issues were young, instable democratic systems, voters inexperienced with democracy who knew no precedence of democracy collapsing, combined with a massive economic crash.

The Empire on the other hand got into power from a completely different precondition. The Galactic Republic had existed for millenia. It was mature beyond belief. It was financially stable, there was no major crisis apart from politics being a little slow. It was just some trade dispute that escalated into full secession, because apparently there has never been a trade war or a separatist movement over the last millenia in a republic that spanned most of the galaxis.

This is where Lucas' US bias shines through. Episode I-III are a case study of what if the US Civil War happened in the 1990s USA, but for no actual reason and then everyone missed that their beloved moderate head of state was secretly Hitler all along.

Here the historical analogy breaks down and it shows that Lucas is primarily an action film maker and not a history scholar.

Trade wars usually don't escalate into real wars because the goal of trade wars is to get more advantageous trade while real wars destroy trade.

The motivations of the trade federation and the separatists are never explained beyond "the world jewish conspiracy wants more power right now".

Because it's mostly an action movie and everything beyond that is just sprinkles on top to justify light saber duels and space fighter dogfights.