Yes WW2 started in 1939, and Pearl Harbor was in december 1941. So it took 2 years for USA to get involved. But USA was such a massive influence, because their industrial output was without comparison bigger than anything else in the world. It was bigger than Europe combined. USA was also a major factor in keeping the trans Atlantic traffic open, and defeating the German submarines. D-Day would have been impossible without USA, as USA was the biggest force. USA also suffered the most losses in operation overlord, with almost twice the casualties and killed as UK. All that is apart from doing by far the most in the effort against Japan.