This history on this is well documented. The US was helping the allies before entering the war by providing weapons and resources. Before being attacked the US was isolationist (like “Trumpists”). It took getting punched in the mouth to muster the resolve to join the war. The attack on Pearl Harbor brought the UK to war with Japan and solidified allied resolve.

https://en.m.wikipedia.org/wiki/Declarations_of_war_during_W...