What are the roles of the united states in the war against japan and the civil war in china in the late 1940's?

In the 1940's japan bombed pearl harbor and we joined ww2 because japan had a allies with Germany so we helped Britain and France fight Germany. Also are troops where spread thin because we where is japan and in Europe.And the civil war happened was when Britain gave Hong Kong. They said no more dictator and this has bin going on for a while now and still china has not settle this problem.