Ten years ago this Sunday, America endured the worst terrorist attack in its history. As Americans and people around the world reflect on the tragic events of Sept. 11, 2001, the following photos help illustrate a decade that was largely defined by 9/11:

The World Trade Center's twin towers, seen here before the 9/11 terrorist attacks, were a prominent feature of the New York City skyline.

The World Trade Center smolders shortly after being struck by hijacked planes on Sept. 11, 2001.

Smoke rises from the Manhattan skyline after the second World Trade Center tower falls.

Dangerous smoke, ash and debris blankets Ground Zero following the towers' collapse.

A rescue worker removes dust from an American flag at St. Paul's Chapel on Sept. 12, 2001.

President George W. Bush speaks through a bullhorn at Ground Zero on Sept. 14, 2001.

The Tribute in Lights memorial illuminates the sky over Manhattan on Sept. 11, 2009.

Commemorative markers glow at the 9/11 Pentagon Memorial on Sept. 11, 2009.

An American flag honors United Airlines Flight 93 on Sept. 11, 2007. The jet crashed into this field in Shanksville, Pa., after passengers overthrew the hijackers.

Guards with the Transportation Security Administration monitor a full-body scanner at Chicago's O'Hare International Airport. U.S. airport security has been vastly upgraded since 9/11.

A U.S. Army soldier patrols at sunrise in Dabay, Afghanistan. The U.S. has been fighting the Taliban and al-Qaeda in Afghanistan since 2001; more than 100,000 U.S. troops are currently stationed there.

President Barack Obama, Vice President Joe Biden, Secretary of State Hillary Clinton and other administration officials watch a live feed of the raid to kill Osama bin Laden on May 1, 2011.

This artist's rendering shows development plans for the World Trade Center site, where new skyscrapers and a memorial are being built to commemorate the fallen towers and nearly 3,000 victims.

A girl looks at a monument for fallen firefighters near Ground Zero on Sept. 5, 2011.

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Photo credits:

World Trade Center before 9/11: Port Authority of New York and New Jersey

Smoking World Trade Center towers: ZUMA Press

Smoke rising from Manhattan skyline: ZUMA Press

Smoke, ash and debris falling on Ground Zero: ZUMA Press

Rescue worker removing dust from American flag: ZUMA Press

President Bush at Ground Zero: White House

Tribute in Lights memorial: Dov Harrington/Flickr

Pentagon memorial: Michael Myers/Flickr

Flight 93 memorial: Josh Buckner/Flickr

TSA agents at O'Hare airport: Scott Olson/Getty Images

U.S. Army soldier in Afghanistan: Chris Hondros/Getty Images

President Obama watching bin Laden raid: White House

Artist's rendering of future WTC site: ZUMA Press

Girl looking at firefighters memorial near Ground Zero: ZUMA Press

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