The U.S. increased spending during the war because of the tremendous amount of money needed to buy weapons, vehicles, ships, and pay the thousands of armed forces members.
WARS ARE EXPENSIVE!!
We fought this war because of deadly threats from Germany and Japan.
Since France was our ally, we needed to help them
AT the beginning of WWII, Russia was ill prepared, in fact Russia only had 13000 operational tanks and maybe 200 fighter planes, the US stepped to the plate and provided 46 percent of all the work materials for Russian in her first 18 months. The US was the banker for the world in those days, as Britian, Russia, Eygpt, and the the rest of europe needed help. So the Lend\lease program was set up. This is the short answer, but we also supplied Austraila, New Zeland, and China with war materials.