The major event that took place at Pearl Harbor on December 7, 1941, was a surprise military attack by the Japanese Imperial Navy against the United States. This attack led to the destruction of significant portions of the U.S. Pacific Fleet, including battleships and aircraft, resulting in thousands of casualties. The event prompted the United States to formally enter World War II, declaring war on Japan the following day. The attack on Pearl Harbor united the American public and shifted the nation from isolationism to active engagement in the global conflict. It became a defining moment in U.S. history, leading to significant military and strategic developments.