How did world war ii impact the economy of the united states?

 When we got into World war II we were still in the Great depression. It brought us out of it. After Pearl Harbor we started drafting men into the army in great numbers. Before this our army was smaller than Holland's. This helped to diminish unemployment. It put the country on a war footing. We began to increase our military capacity by building armaments esp. planes. Nothing can help an economy like war. Because so many men were drafted into the army, women were employed to do work that was previously only done by men, like welding and producing armaments. Our technology especially in weaponry was far behind that of our enemies at the beginning of the war. We didn't have planes as good as Japan or Germany at the beginning. Our Sherman tanks couldn't compare with the German tanks. We had to catch up. So since WWII and the beginning of the Cold War we feel that we cannot afford not to have the best weapons. Military spending has been an important part of our economy ever since. 
The military tactics were different in WWI and WWII. In WWI it was mostly trench warfare. The Second World War saw the use of "blitzkreig" warfare. I hope this has been a help to you. 
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World War II brought the economy out of the Great Depression. Once the war started, the government began putting people to work in factories and as soldiers. After the war, the Cold War was starting and there was a Space Race to win. There was still plenty of government spending.

Most all wars are good for the economy in that it creates jobs for people that are not fighting the war. It also created work for factories in that they had to gear up and produce goods to be used in fighting the war.