Why does the US get involved in the war?

Answer 1

The US were helping Britain without getting personally involved - lease lend being one example of helping on the quiet. Then the Japanese attacked Pearl Harbour and directly brought America into the war. America declared war on Japan, and then Germany declared war on America (as part of a treaty between Germany and Japan). It was the industrial might, and the mass numbers of American troops available that eventually swung the war into the Allies favour.