so, I have seen alot of history documentaries, books, movies, games, web bloggs, ect. But still i don't understand why china almost always splits up. the most recent one was probobly the one after or during WW1 where china became a republic but there where also alot of warlords fighting for china to restore the empire. why does this hapen so much and when it does it last a long time.
The short answer is because it's a massive diverse area with a huge population and a lot of people seeking different goals. China, to many outside the area, seems like this homogeneous place where everyone's more or less on the same page, speaking the same language, after the same goals. But in reality this can't be further from the truth.
Officially, China has 56 ethnic groups. These are referred to as Nationalities (minzu). However like all cases of official recognition of ethnicity in East/Southeast Asia, it's not even close to accurate. As an example, a single one of these, Gaoshan, actually includes what are more reasonably 16 different ethnic groups with their own cultures and languages, but which have been combined into a single "ethnicity" by the government. Others, such as Yi, likewise include a huge number of languages and cultures. Zang, also known as "Tibetan", includes an absolutely absurd range of diversity. Under Zang you will find speakers of Amdo, Kham, and more traditionally Tibetan langauges, but also a wide range of speakers of Na varieties, Qiang varieties, rGyalrong and many more. Then on top of that, Han, the majority group, is also arguably multiple different groups which have been combined as the majority, but a) the groups included in Han today weren't always included by everyone and b) like "white", it's more a "everyone who isn't 'the Other'" kind of grouping. A lot of what people think they know about even Chinese language is built on myth. For example written Cantonese is not the same language at all as written Mandarin, but the myth persists that it's just variations on a theme.
A really good analogy is Europe. Why does Europe keep splitting? Sure they've been kinda unified in the past, but they also have long periods of not being unified. In many significant ways, it's analogous with China. You've got a huge area with a linguistically and ethnically diverse population. Keeping that together and keeping everyone happy is not an easy thing, especially when it's a system where there are ethnically based strata, and a lot of people remaining unrecognised within this system.