Claiming a territory against the will of its people is exactly what imperialism is

In case you're interested in learning more about imperialism from a Marxist perspective, I'd recommend Lenin's Imperialism, the Highest Stage of Capitalism. You don't have to agree with it, but I don't believe you can have a complete understanding of the way Global South countries have been exploited and subjugated in the last century or so without being aware of the arguments made in this book. The fact that so many of the post WW2 anti-colonial liberation movements were ML or MLM should tell you something.

Also, China doesn't have to claim anything. Elsewhere in this thread people have already pointed out that under International Law Chinese Taipei is already part of China. No country in the world recognizes the ROC or Taiwan as an independent country. To the extent that the populace of the island has a position on this, it's split between wanting to retain a special status within China or separatism. And let's be real here, no country in the world is down with separatist movements. Ask the Kurds, or the Basques, or the Catalonians, or the Chechens etc etc. Hell, let Texas try and secede and see what happens. Realistically, only once Capitalism and Nationalism are both dead and forgotten can humanity start to move away from this geopolitical reality.