The sinking of the Lusitania and sinking of American vessels. Also a telegram (no one knows if it is was real of fake) was sent from the British Empire to America, stating that they intercepted it from the Germans. The telegram read, "Mexico, if you join the axis powers you can have all the land you want in the United States,".
Of course no one knows if that is true or not.
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It joined by declaring war after the Japanese Attack on the US Fleet at Pearl Harbor.
did us join war in1914 yes or no
The League of Nations
The Japanese attack on Pearl Harbor.
People in the US felt that Europe was Europe's problem.