What brought the US into war?
Officially, bombing by Japan on American Naval fleet at Pearl Harbor in 1941. Unofficially, the President was supplying aide to Great Britain with arms, supplies to fight the Nazi's in 1940 although we kept the stance we were neutral for public appearances.
The Mexican War 1846-1848 brought the United States; California, Nevada, Utah, New Mexico, and Arizona. WW2 brought the world into the atomic age. The Prussian War created Germany in 1871. The Revolutionary War created the USA. The US Civil War saved the Union (US). The Vietnam War gave the communists their only true victory during the cold war.
World War II is actually what completely brought the United States out of the Great Depression. In the beginning of the war, similar to what we did in WWI, we sold weapons, ammunition, and other war materials to the Allies in Europe. This greatly helped out economy and brought us out of the Depression.