Prior to Edelgard starting a war though, we also have an unbeatable magic Professor, countless students negatively influenced by the status quo eager to change things, and the future rulers of every continent of the nation attending school together at the same time, each of those rulers possessing a very strong desire to institute reform. And two of those rulers are siblings and friends. Change was absolutely possible even without a war. Guaranteed? No, but the pieces were in play in a way that I don't think any previous generation would have been able to say the same of.