Since it’s not a Code Geass post ending discussion thread and you tagged your post as spoiler, I would try to make it as spoilerfree as possible. Besides I'll try to keep it as short and simple(tho it turned out quite big for something intending to be short & full of spoilers) because to be fair, whether u agree or not this is a pretty big topic and I'm not a volunteer,specially now I don't take part is discussion because of being busy with irl stuffs.
[Code Geass Ending discussion spoiler](/s>How exactly does it make sense that there would be peace
When exactly does the show say that eternal peace would be there just because some tyrant political leader died? The epilogue itself said that there are still problems left and the world by no means perfect as realistically it shouldn't be. Infact the guy whom you’re blaming here said earlier that he wasn't aiming for a perfectly peaceful world but a better one by breaking the garbage system it was into where every country was offensive toward others..This is the most superficial explanation, and it already contradicts with your view over the ending. It's not about " woah tyrant ruler died for peace, yay". there are whole scenarios and stings which forms the peace however it’s peace in a sense that the three superpowers were able to break free from the cycle of hatred they were previously into. They are all now great countries who fought against a tyrant who lublicly declared to control world alone with his dictatorship and almost did it successfully before an iconic figure killed him.
Yes there would be peace but that completely depends on people. But after getting torn through war again and again people themself understand what they should do. Besides the military force isn’t under their control so that they would start invading countries once again.There are a proper structure and reasoning behind that. Some small conflict would always be there, it’s just nature of people. none can stop it but people can look forward this change that was made and they can try for better. That's the main point, casually saying.
Especially when it is a fully militarised world that has experienced, and by all accounts not particularly nice, military leaders.
It's not fully militarized world now. There are still some superpowers that follow fragments or atleast some level of democracy or principality . And the world can only be considered as fully militarized when you take account UFN but UFN runs by democracy and decision depends on whole world's political leaders.
And most of the "not-nice" leaders who militarized the world previously & invaded one country over either dead and got replaced by actual leaders who sympathized with people at that point or the chief ones like PM of Britannia was under influence of geass of Lelouch so that other leader's demise wouldn’t create power vacuum apparently which in the end helps Zero Requiem.
The complete opposite to peace would happen: it would be all out war all over the shop.
And who exactly have might forge war at this point unless they are some mini independent country that's not under control of UFN? Even those mini Countries are pretty much unmatched to Black Knight in Military Might. This is also an isse that was introduced by the new AU movie, not like the author don't even know what might happen because of possible change after Zero Requiem . Not only the world is filled with lots of humane problems from past wars and Britannian past and currently deceased Emperor Lelouch's rulling but also it’s under one military might, Black Knight which under authorities of UFN & administrated by Zero who is a valid part of this new world deceased emperor was talking about.
So it’s not possible to forge war at this point, you can argue that UFN might break their unity but that's why new Zero is there.)