What significant event occurred on December 7, 1941?

The attack on Pearl Harbor on December 7, 1941, was a pivotal moment that led to the United States entering World War II. This surprise military strike by the Imperial Japanese Navy against the U.S. naval base at Pearl Harbor in Hawaii resulted in significant loss of life and damage to the Pacific Fleet. The event galvanized American public opinion, leading to the declaration of war against Japan the following day and marking a major turning point in the conflict.

In the context of World War II, the attack is often viewed as a catalyst that transformed the U.S. from a nation of isolationism into an active participant in the global war, thereby reshaping the dynamics of international relations for years to come. The other options refer to different historical events: the beginning of World War II is generally marked by the German invasion of Poland in 1939, the signing of the Armistice pertains to the end of World War I, and the Manhattan Project was a scientific endeavor that began later in the war, focused on developing nuclear weapons.