Question

Was the us justified in joining the allied powers and declaring war on Germany???

Answer 1

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

Answer 2

The U.S. was justified in declaring war on Germany due to Germany firing a torpedo on an U.S. passenger ship (Lusitania)