On Sept. 11, 2001, 19 al-Qaida hijackers seized control of four commercial airliners in the United States. Two planes were flown into New York City's World Trade Center, a third struck the Pentagon in Virginia and a fourth crashed in Pennsylvania after passengers attempted to overcome the hijackers. Nearly 3,000 people were killed.

The attacks became one of the defining events of the 21st century, fundamentally changing U.S. national security policy and influencing international conflicts for years. Today, memorials and ceremonies are held across the United States and elsewhere to remember those who died and recognize the lasting impact of the attacks.

The National September 11 Memorial at the World Trade Center site in New York City.

Chile's democratically elected government was overthrown on Sept. 11, 1973, when Chilean armed forces led by Gen. Augusto Pinochet launched a military coup against President Salvador Allende. Military forces attacked the presidential palace in Santiago as the government collapsed.

Allende died during the assault, and Pinochet emerged as the leader of a military government that ruled Chile for the next 17 years. The coup marked a major turning point in Chilean history and became one of the most significant political events in Latin America during the Cold War.

Chilean President Salvador Allende during the period surrounding the 1973 military coup..

Alexander Hamilton was appointed the first U.S. secretary of the Treasury on Sept. 11, 1789, by President George Washington. The appointment came as the newly established U.S. government worked to create the financial institutions needed to manage the nation's finances.

Hamilton became one of the most influential figures in the early American government, helping establish policies that strengthened federal financial authority. He remained Treasury secretary until 1795 and played a major role in shaping the economic foundations of the United States.

Alexander Hamilton, the first U.S. secretary of the Treasury, was appointed to the position on Sept. 11, 1789.