Private rescue efforts have been ad hoc, scattershot and, at times, divisive. Mr. Prince, whose Blackwater guards were convicted of killing civilians in 2014 while providing security for Americans during the Iraq war, said he was charging each passenger $6,500 to get them safely into the airport and on a plane, and it would cost extra to get people who have been trapped in their homes to the airport. It remained unclear whether Mr. Prince had the wherewithal to carry out his plans.

Most of these evacuation efforts, however, are driven by genuine empathy for Afghan friends and colleagues facing Taliban retribution. The biggest challenge the groups faced was getting people with seats on the charter planes through the gauntlet of Taliban checkpoints, crushing crowds at the airport entrances, and U.S. forces who refuse to let manifested passengers in.

“It’s a combination of tragic, surreal and apocalyptic,” said Stacia George, director of the Carter Center’s Conflict Resolution Program, who has been working round-the-clock to get people out of Kabul. “It’s so frustrating to get high-risk people up to the gate and have them risking their lives to go there and you still can’t get them through. It’s a disaster in slow- and fast-motion.”