What major event began on December 7, 1941?

The correct answer is the attack on Pearl Harbor, which occurred on December 7, 1941. This surprise military strike by the Imperial Japanese Navy Air Service targeted the United States naval base at Pearl Harbor in Hawaii. The attack resulted in significant losses, including the sinking of battleships, damage to aircraft, and the death of over 2,400 Americans.

This event is often regarded as the catalyst that led the United States to formally enter World War II, as it galvanized public opinion and spurred the U.S. government into action against Japan. Following the attack, President Franklin D. Roosevelt delivered his famous "Day of Infamy" speech on December 8, calling for a declaration of war against Japan, which Congress swiftly granted.

In contrast, the other options refer to events that occurred at different times in history: World War I began in 1914, the Treaty of Versailles was signed in 1919 to formally end that conflict, and the Apollo program, which aimed at landing humans on the Moon, began in the 1960s. Therefore, the significance and timing of the attack on Pearl Harbor are what set it apart as a key historical event on December 7, 1941.