Answer:
This a very interesting and general question.
Step-by-step explanation:
The US officially entered World War 2 on December 11, 1941. Mobilization began when the United States declared war on Japan on December 8, 1941, one day after the attacks on Pearl Harbor.
This move caused Germany, an ally of Japan at the time, to declare war on the United States on December 11th, sucking the US into the European Theater of this global conflict, and taking the US, in just four short days, from a peacetime nation to one that was preparing for all-out war with two enemies on opposite sides of the globe.
Although formal declarations of war did not come until 1941, one could argue that the US had been involved in WWII for some time already, since 1939, despite the country’s self-proclaimed neutrality. It had played a role by supplying Germany’s opponents — which, by 1940, after the Fall of France to Hitler and Nazi Germany, included pretty much only Great Britain — with supplies for the war effort.