World War 1:
Before the U.S. declared war on April 6th, 1917, the U.S. had desperately tried to stay neutral, but ties to Britain, propaganda, the sinking of ships by German U-boats, and a German attempt in the Zimmermann Note to get Mexico to declare war on the U.S. pushed the U.S. to getting involved.
World War 2:
Although the war began with Nazi Germany's attack on Poland in September 1939, the United States did not enter the war until after the Japanese bombed the American fleet in Pearl Harbor, Hawaii, on December 7, 1941. The American people didn’t want to get involved in a war seeing how they were in an economic depression. However, when Japanese air squadrons bombed an American Naval Base, the American people were furious. The following day, President Franklin Delano Roosevelt declared war on Japan, thus getting America involved in WW2.