The reason why public opinion shifted so dramatically from 1940 to 1941 on going to war against the Axis Powers was the attack on Pearl Harbor.
Even though the second world war started in 1939, most Americans did not want to go to war with the Axis powers in 1940. About 88% of Americans favored allowing Europe to deal with its war.
This all changed in December 1941. This was as a result of a Japanese attack on Pearl Harbor in Hawaii. Thousands of Americans died and because Japan was an Axis power, public opinion shifted dramatically to supporting going to war against the Axis.
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