The major event that distinguished U.S. involvement in World War II was the attack on Pearl Harbor on December 7, 1941. This surprise military strike by the Imperial Japanese Navy on the U.S. naval base in Hawaii, which led to the destruction of battleships and the loss of more than 2,400 lives, propelled the United States into the war. In response, President Franklin D. Roosevelt declared war on Japan the following day, marking a significant shift in U.S. foreign policy from isolationism to international engagement. This event galvanized American public opinion and mobilized military resources for the subsequent Allied campaigns.