The quiet departure from Bagram Air Base brought an effective end to the longest war in US history. It came as the Taliban insurgency ramps up its offensive throughout the country after peace talks sputtered.

The Pentagon said the turnover of Bagram to Afghan security forces was a "key milestone" in the withdrawal, but insisted the U.S. military still has the authority to protect Afghan forces.

"Those authorities still exist," Pentagon spokesman John Kirby told reporters. He did not give a timeline for when they might end.