Question

why did america join 2nd world war afterwwards​

Answer 1

On 11 December 1941, four days after the Japanese attack on Pearl Harbor and the United States declaration of war against the Japanese Empire, Nazi Germany declared war against the United States, in response to what was claimed to be a series of provocations by the United States government when the US was still officially neutral during World War II. The decision to declare war was made by Adolf Hitler, apparently offhand, almost without consultation.

Answer 2

America joined the 2nd World War to take revenge on Japan because Japan blasted the Pearl Harbor.