To explain their empire and concur more land
The alliance between Great Britain, France, and the United States during WWII was called the Allies. If you're truly in High School; you should know this by now. But I'm not judging-so don't take it the wrong way.
The Germans sank a British ship which had American passengers. This was the first step. After this due to the unrestricted submarine warfare more US ships were sunk by German U-boats.