Franklin Delano Roosevelt. When he was elected he was a disabled person. He needed a wheelchair after contracting polio. He not only didn't give up. He ran for president in 1932! While in office he drastically decreased unemployment from 25 percent to 2 percent. He carefully prepared America to enter the battle during World War II by increasing the defense budget and converting the U.S. to a military economy. That's why we won this war.
Unemployment decreased because of the war. and defense spending really didn't go up until the start of WWII. And if you look at armaments spending in 1933, 1934, 1935 you'll see he spending less on armaments. Also don't forget we were not prepared for war that it took us almost a year to get up to speed. We spent much of 1939 and 1941, ramping up “preparedness”.
We won partly because there was a huge ocean between us the the fighting, meaning that we weren't being bombed and our industry remained intact, unlike the other waring countries. We also had resources that we could call upon within our own boarders. We also won in part because England held out long enough for us to get war production up and running. Roosevelt didn't end unemployment nor the depression, that was the result of WWII.
The war was won, because to the end of it Germany was in multiple front war, which its economy and army cloudn´t handle.