The question with China is if it reunites Taiwan will the PRC stop there? The incursions into the South China Sea taking disputed territory from.the Philippines & co suggests not. Just like Russia won't stop with the claimed territories in the Ukraine if a peace deal was signed.
That's why the world needs a strong USA to counteract China & Russia, not to shrivel up, become weak & isolationist. That's the real threat from a Trump Presidency not what happens domestically in the USA.