How to govern cities and states that were previously self-governing
Now let's talk about conquering places used to governing themselves, like republics or democracies.
There are three ways to do this: (1) burn it all down, (2) go live there, and (3) basically leave them alone but make them pay taxes and create a small government.
Got it? Now forget the last two, because those aren't going to work. You need to destroy these places.
Republics are too feisty, and they don't seem to like the idea of being conquered by kings, oddly enough. If you don't burn it all down, they'll come back to bite you when you least expect it, even 100 years later.