Back in 2001, the US was responding to the 9/11 attacks on New York and Washington, in which nearly 3,000 people were killed. Officials identified Islamist militant group al-Qaeda, and its leader Osama Bin Laden, as responsible.
Bin Laden was in Afghanistan, under the protection of the Taliban, the Islamists who had been in power since 1996.
When they refused to hand him over, the US intervened militarily, quickly removing the Taliban and vowing to support democracy and eliminate the terrorist threat.
The militants slipped away and later regrouped.
Nato allies had joined the US and a new Afghan government took over in 2004 but deadly Taliban attacks continued. President Barack Obama's "troop surge" in 2009 helped push back the Taliban but it was not long term.
In 2014, at the end of what was the bloodiest year since 2001, Nato's international forces ended their combat mission, leaving responsibility for security to the Afghan army.
That gave the Taliban momentum and they seized more territory.
Peace talks between the US and the Taliban started tentatively, with the Afghan government pretty much uninvolved, and the agreement on a withdrawal came in February 2020 in Qatar.
The US-Taliban deal did not stop the Taliban attacks - they switched their focus instead to Afghan security forces and civilians, and targeted assassinations. Their areas of control grew.