While conquering something obviously makes it yours (if you can defend it, you own it), we certainly shouldn't let China take Taiwan. Ultimately most of them want to remain independent, but they've stated that if forced to choose, they'd rather join America than China. And due to a few WWII treaties, we have to a) defend them from outside invasion and b) acknowledge their choice, should they decide to annex themselves into a US territory.
And while I'd have no problem with Germany taking Austria, I don't think the Germans want it at this point.