The whole thing is worth reading, here:
Biden could have easily said the Taliban had reneged on their agreement
with the United States so he could continue to keep a relatively small
US military force in Afghanistan to advise and assist the Afghan Army
and to support the Afghan Air Force to thwart Taliban advances.
... the American presence in Afghanistan had shrunk to only 2,500 troops --
particularly few for a force of 1.3 million active-duty US service
personnel. That small force helped to sustain the Afghan military
physically and psychologically, not least with close air support.
Now,
the Biden administration unilaterally has pulled the plug on the US
troop presence in Afghanistan, which cratered morale among the Afghan
military and population. It also precipitated thousands of
Western-allied soldiers to head for the exits, as well as the many
thousands of contractors in Afghanistan that were, among other things,
keeping the Afghan Air Force aloft.
And now the white flags of the Taliban flutter all over Afghanistan. It did not need to be this way.