image source, EPAimage caption, Thousands of Afghans sought to leave through Kabul airport, which was guarded by US troops
The top US general and the secretary of defence are being questioned in Congress over the military withdrawal from Afghanistan last month.
US troops had to accelerate their withdrawal after Kabul fell to the Taliban on 15 August.
Senator and committee leader Jack Reed said that lawmakers want to understand whether the US "missed indicators" of the government's collapse.
The US has said it will now move toward counter-terrorism missions.
The hearing, held by the Senate armed services committee, comes weeks after a chaotic withdrawal at Kabul airport as foreign powers sought to get their citizens home and thousands of desperate Afghans begged for rescue.
A suicide attack killed 182 people during the withdrawal operation. Thirteen US service personnel and at least 169 Afghans were killed by the airport gate on 26 August.
Tuesday's hearing began with opening testimony from Defence Secretary Lloyd Austin followed by Gen Mark Milley.
Another US general, Kenneth McKenzie, will also appear. As head of US Central Command, he oversaw the withdrawal from Afghanistan.
US troops first entered Afghanistan in late 2001, shortly after the 9/11 attacks. By the time they left, the US had spent about $985bn (£724bn) and deployed tens of thousands of troops, peaking at 110,000 in 2011.
In the weeks between the fall of Kabul and the withdrawal deadline of 31 August, the US evacuated its remaining 4,000 troops. It is also taking about 50,000 Afghan refugees who were airlifted out of Kabul.
As many as 20 people died in the crowds which gathered at the airport in the days after the Taliban takeover.
Who is Gen Mark Milley?
- He's Joe Biden's top military adviser – the chairman of the Joint Chiefs of Staff (the committee of the eight highest-ranking military officials)
- He's not part of the chain of command with the military and does not order US forces
- However, he is the link between the White House and the Pentagon
- He was a four-star officer and the Army Chief of Staff before being appointed to lead the Joint Chiefs of Staff in October 2019
- ANALYSIS: How much has the Afghanistan war cost?
- REPORT FROM KABUL: A new order begins under Taliban rule
Gen Milley is likely to face tough questioning on Tuesday, especially from Republicans, who have called for him to be sacked.
He and Gen McKenzie will probably be asked about a US drone strike in Kabul on 29 August which killed 10 innocent members of a single family.
After the attack, Gen McKenzie said US intelligence had tracked a car belonging to one member of the family, an aid worker, believing it was linked to a branch of the Islamic State (IS) group.
Gen Milley originally described the attack as a "righteous strike". After the Pentagon established that the dead were all civilians, he backtracked, admitting that he had spoken too soon.
It emerged recently that he held phone conversations with the Chinese military following concerns about then-President Donald Trump.
The phone calls were revealed in a book by journalist Bob Woodward, who also said Gen Milley had told his staff that if Mr Trump ordered a nuclear strike, then he would have to confirm it before it was carried out.
A spokesman for Gen Milley defended the calls, saying they were part of his duty to maintain "strategic stability".
Top Republican Senator Marco Rubio described this as "treasonous".