Over the years, the nation has come together across the country to commemorate the anniversary of the September 11 terrorist attacks.
Take a look at some of the photos of the U.S. Presidents paying their tributes.

The events of September 11, 2001 permanently changed America's perception of security and prompted then-President George W. Bush to declare war on terrorism and invade Afghanistan after the deadliest attack on American soil since Pearl Harbor in 1941.

U.S.-led coalition forces have knocked the Taliban from power in Afghanistan. The war there continues, although earlier this year the U.S. and Taliban signed an agreement aimed at ending the conflict.

The United States searched for al-Qaida leader Osama bin Laden for years after the terrorist attacks, and in May 2011 a team of U.S. Navy Seals raided a compound in Pakistan where they shot him dead.

Memorials for those killed in the attacks now stand at sites in New York, Virginia and Pennsylvania. The Pentagon crash site was quickly rebuilt after the attacks. A new tower at the World Trade Center site took longer to construct, but now rises above the Manhattan skyline as the tallest building in the United States.

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U.S. President George W. Bush and first lady Laura Bush lay a wreath at the crash site of United Flight 93 in Shanksville, Pennsylvania, September 11, 2002.

U.S. President George W. Bush, first lady Laura Bush, Vice President Dick Cheney and his wife Lynne place their hands on their hearts as after observing a moment of silence at 8:46 a.m. on the South Lawn of the White House September 11, 2004.

U.S. President George W. Bush and first lady Laura Bush attend a memorial service and wreath laying ceremony on September 11, 2006 at the crash site of flight United 93, inside the impact zone, in Shanksville, Pennsylvania, commemorating the fifth anniversary of the terrorist attacks.

U.S. President Barack Obama, Chairman of the Joint Chiefs Adm. Mike Mullen and U.S. Secretary of Defense Robert Gates attend a Pentagon event in Washington on September 11, 2010 to the ninth anniversary.

U.S. President Barack Obama, first lady Michelle Obama, and Defense Secretary Leon Panetta pay their respects in front of a wreath during a ceremony commemorating the 11th anniversary of the attacks at the Pentagon in Washington, D.C. on September 11, 2012.

President Barack Obama, first lady Michelle Obama, and Vice President Joe Biden, stand on the South Lawn of the White House with White House staff members on September 11, 2014 as they observe a moment of silence to mark the 13th anniversary of the attacks.

U.S. President Barack Obama places a wreath during a ceremony marking the 15th anniversary of the attacks at the Pentagon in Washington, D.C. on September 11, 2016.

President Donald Trump and first lady Melania Trump stand for a moment of silence to mark the anniversary of the Sept. 11 terrorist attacks, on the South Lawn of the White House, Monday, September 11, 2017.

President Donald Trump and first lady Melania Trump, stand at the Flight 93 Memorial, Tuesday, September 11, 2018, in Shanksville, Pennsylvania.

U.S. President Donald Trump and first lady Melania Trump observe a moment of silence on the South Lawn of the White House on September 11, 2019 to mark the 18th anniversary of the attacks.