After facing an oil embargo by the United States and its own reserve oil supply about to run short, the Japanese government decided to take action and execute a plan developed by the military branch largely lead by Osami Nagano and Isoroku Yamamoto to bomb the United States naval base in Hawaii, thereby bringing the United States to World War II on the side of the Allies. On 4 September 1941, the Japanese Cabinet met to consider the war plans prepared by Imperial General Headquarters, and decided:

Our Empire, for the purpose of self-defense and self-preservation, will complete preparations for war ... [and is] ... resolved to go to war with the United States, Great Britain and the Netherlands if necessary. Our Empire will concurrently take all possible diplomatic measures vis-a-vis the United States and Great Britain, and thereby endeavor to obtain our objectives ... In the event that there is no prospect of our demands being met by the first ten days of October through the diplomatic negotiations mentioned above, we will immediately decide to commence hostilities against the United States, Britain and the Netherlands.

The Imperial Japanese Navy made its surprise attack on Pearl Harbor, Oahu, Hawaii, on the Sunday morning of December 7, 1941. The Pacific Fleet of the United States Navy and its defending Army Air Forces and Marine air forces sustained significant losses. The primary objective of the attack was to incapacitate the United States long enough for Japan to establish its long-planned Southeast Asian empire and defensible buffer zones. The U.S. public saw the attack as a treacherous act and rallied against the Empire of Japan. The United States entered the European Theatre and Pacific Theater in full force. Four days later, Adolf Hitler of Nazi Germany declared war on the United States, bringing the separate conflicts into a cohesive conflict.

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