How Did World War II Affect the U.S. Economy?

What factors influenced the U.S. government to enter the war against Japan, Germany, and Italy in December 1941? What types of fanatical warfare did the Japanese and Germans rely upon? What measures did the U.S. military take to knock Germany and Japan out of the war? How did the U.S. economy improve as a result of involvement in World War II? Provide statistics to support your answer.