1917 smart
1941
1917.
Un-restricted submarine warfare. If it wasn't for that, the US would not have entered the war. As it was, the US was only in it for a year anyway...it ended in 1918 (the US entered the war in 1917).
As soon as the US entered the war in December 1941
April 1917
1909 to 1944 The US entered World War 2 in December of 1941.
The US entered the war on April 6,1917
The US entered World War 1 on April 6, 1917. It remained in the war until it ended about a year and a half later.
The allies entered the war in 1914; the US entered the war in 1917.
the Atlantic Ocean.
The US entered in 1941, after the Pearl Harbor bombing. I'm guessing this is where you are from?