Question

why did the us enter ww2

Answer 1

The USA didn't like the Japanese attacking the Chinese in Manchuria... so they imposed trade restrictions on Japan. Japan in turn thought okay, if we can't get oil etc from the USA we'll attack the East Indies and get it from them instead... but they figured if they did that they'd face military opposition from the USA, more than likely delivered by America's Navy.
The Japanese solution - take out the US Navy at Pearl Harbour and neutralize it so it can't be used against them... which they did. The result, America in turn declaring war on Japan... and Germany supporting their Axis partner by declaring war on the USA a few days later.