Most countries dont need "justification" for joining a war, typically the only justification is a decision backing up allies, and under this definition, the US was justified, Japan had also initiated aggression againts the US, giving the US an incentive to join the war, so yes, its likely that they were justified in joining the war, for several reasons
Tibet thinks that it ought to be an independent & free country & has made international representation in the United Nations that China has occupied its territory.
China thinks that Tibet is under its territory and it has right to control it
