Vietnam: the majority of the people of Vietnam didn't want the US there, the US was interfering in a civil war it shouldn't be meddling in.
Iraq: same thing, the US wasn't wanted and didn't have a reason to meddle.
Taiwan: the people of Taiwan don't want China there and China shouldn't invade. The US should also not invade.
People have a right to self determination. If China doesn't control Taiwan and the people of Taiwan want to keep the same government they have, then they should be allowed to. China invading Taiwan would be an unjustified imperialist act. Taiwan has a right to call upon its allies and friends for help if invaded and the alliances and friendships should be honored by those allies.