This is a cliché in Japanese fiction that is irritatingly common.
It is based on the idea that war is fundamentally bad and, if we all just put aside our hatred for each-other, we can live in eternal peace and harmony.

Now, there are various reasons why this is irritating, though this being a cliché and not a trope should indicate that it’s generally poorly implemented.
Let’s start with the premise. Wars are not fought exclusively because we hate each-other. Many wars are fought for ideological reasons…or for resources. There are even people who think that wars are exclusively fought for resources rather than ideology/hatred. I don’t think that’s the case, but that’s not relevant right now.
The point is that wars are fought for a variety of reasons, so this assumption is quite naïve.
The next issue is that it generally ignores real-world considerations when these differences are put aside. IE…the fact that we are choosing not to hate each-other doesn’t change the fact that resources are still scarce, for example. Or it doesn’t change the fact that not everyone wants to live in the same kind of society. Or any number of other things.
This kind of world is destined to eventually fall back into a state of war at some point.

ao6 is one of the more vexing examples of this trope at play. Not only does it suggest this, but it has the gall to suggest that the conflict depicted is morally neutral. I would suggest reading the review for an explanation.

Code Geass is a…I don’t know whether interesting or not example.
Code Geass understands that ‘hatred’ isn’t all the drives warfare…but it clearly assumes that hatred plays a big part.
The Zero Requiem plan is, in fact, directly dependent on this assumption to work. Though it’s kinda quirky because of how it’s all set up…yeah, it’s just weird. Like all of Code Geass I guess. I’d need more time to think about it.

I added Cross Ange: Tenshi to Ryū no Rondo to this list at some point, but forgot why. I think it’s because the assumption is there though it doesn’t quite go into this cliché’s territory as far as I remember.

le is a quirky example as the first season arguably depicts this cliché correctly, with proper set-up and pay-off. The war depicted is somewhat pointless, yet also over scarce resources and putting aside their differences really is the best option in their situation (otherwise they’d all die). The ending, in the end, really is mostly happy.
However, the second season (Gin’yoku no Fam) actually throws this out the window and goes all-in on the cliché.

yakuneve is another quirky example because it’s correct on the one hand but also incorrect on the other. Though further explanation veers into spoiler territory so I recommend reading the manga…or just reading my review if you’re lazy.