The US military knew there was going to be an attack at the Kabul Airport last week.
On Wednesday, August 25th the US State Department issued a warning to US nationals to avoid traveling to Hamid Karzai Airport in Kabul, Afghanistan. The US told those waiting outside to leave immediately.
On Thursday, August 26th a suicide bomber blew himself up near the Abbey Gate at the Kabul Airport killing 13 Americans: 11 US Marines, a Navy Hospitalman, and an Army Staff Sgt.
The Taliban allowed the bomber to get through the perimeter. The Marines did not allow the bomber to get through the inside perimeter. He was outside the gate when he detonated his explosive vest by the crowded Abbey Gate at the Kabul Airport.