The U.S. didn't want to participate in the Second World War from the start. As in the First World War they waited until it had something to do with them as they were not directly allied or had any treaties signed with England or any European countries. U.S. President at the time Franklin D. Roosevelt did however supply the British Army with weapons such as the Thompson which W. Churchill was a big fan of.
Only until the Japanese attack on Pearl Harbor on December 7th, 1941 did the U.S. actually get involved in World War II. A short time later Nazi Germany declared war on the U.S. as well. The U.S. which at the time was brought out of a massive depression with this war was able to build enough troops to fight in the Pacific and in Europe. The U.S. Marines fought through Singapore up to Iwo Jima eradicating the Japanese and the U.S. Army went through Africa then landed on D.Day in 1944 to eliminate all German resistance until Berlin was finally taken by Soviet troops who were massed in the millions to take any German lives if possible.
The U.S. was ideally the smartest country in the war being one of the only ones not to jump in right at the start and lose more people. It's also considered when President Roosevelt died and President Harry S. Truman came to office and dropped two atomic bombs on Japan it was a figure of saving American lives as a land takeover of Japan would take a huge toll on American lives.
I hope this answers your question.