Answer: The Japanese attack on Perl Harbour
Explanation: The United States did not intend to enter WWII by adhering to its isolationist policies, and all political disputes with Japan sought to smooth diplomatically. That is why the attack on Pearl Harbour itself was a surprise to Americans, and that surprise was one of the factors that contributed to the catastrophic casualties. The attack on Pearl Harbour was something that could not be ignored and the US entered the WWII on the Allies' side.