The United States of America joined in the World War II after 7th of December, 1941; the day on which it's Pearl Harbor was attacked. Prior to this America had stayed away from the war and wasn't a part of either the Allied or the Axis powers. However, when Japan destroyed almost the entire American navy with a surprise bombing of Pearl Harbor which was of its primary naval bases, the U.S joined in fighting along side France and Britain.
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The longest war the United States ever entered was the Vietnam War.
The US entered World War 1 on April 6, 1917. The war ended about one and a half years later.
The US entered when it declared war on Germany on April 6th 1917. So they entered late.
The US entered WW1 in 1917. The US entered WW2 in 1941.
The United States entered World War II after the Japanese attack on Pearl Harbour on 7th December 1941. The US declared war on the following day, 8th December 1941.