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.

Something that all of these stories inevitably struggle with is the question “what happens next?” if they continue after the Japanese Kumbaya ending.

From my experience…confusion. This is because the writer seems to realize that stuff just doesn’t add up, so they struggle to answer the question properly. The Japanese Kumbaya is meant to be functionally identical to a “Happily Ever After…” which is probably why the struggle exists: it wasn’t ‘designed’ with the expectation of continuation.

However, I think it can go into a few directions:

• The plot will revolve around someone attempting to go back to the state of war.^A)
• The world just straight-up devolves back into war.
• The story starts meandering.
• The same thing happens again.^B)
• The story starts to focus on something vaguely related but ultimately distinct.^C)

Though I haven’t observed this that much, since stories don’t tend to continue after the fact that often.

I thought of this recently, but realized that…maybe there is a situation where a line of reasoning can lead you to conclude this actually makes sense.

I think that this might be due to Japan’s homogenous and high-trust society.

This perspective – that we’re all basically the same and should just set our petty differences aside – make the most sense if everyone is already on the same page…like in Japan itself.

Allison is one of the extremely naïve examples of this cliché as it literally assumes that all conflict boils down to ideology/hatred.
The first book ends with a Cold War coming to an end.
Though…this is also a bit of a quirky example, since the world depicted is so obviously artificial. It’s written like this on purpose, but I don’t think that excuses the naïveté.

Since I just released the article, I’ll elaborate a little more before I bother re-writing this section. The world depicted in Allison is extremely artificial…like it was purpose-built just to communicate this cliché/theme. I suspect that it’s intentional.
This also has the negative domino effect of making everything after the first volume of the series (and its sequel) less believable…because a world like that just can’t function.

Ao no 6-Gō 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.

Appleseed depicts this in a way, just that it goes about it differently. It still assumes that all war boils down to hatred and whatnot…but instead of depicting a world where humans set aside their differences…Appleseed creates an entirely new race of artificial humans – bioroids – who have their negative emotions suppressed and, as a result, they are immune to committing crime or war.

This example is quirky because…well, Appleseed doesn’t claim that humans can be like this. So I think it actually manages to avoid the cliché by thinking about it a little more.

Code Geass is a…I don’t know whether interesting or not example.
Code Geass understands that ‘hatred’ isn’t all that 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.

LASTEXILE 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.

I realized I named this cliché on the dedicated article and, since I already explain it there, I’m not gonna repeat myself. Just read the review.

Yakusoku no Neverland 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.