But for all of Mr. Biden’s insistence on sticking to his withdrawal deadline, neither the Department of Homeland Security nor the State Department has been able to ratchet up the vetting and processing times to the levels needed to meet demand.

A U.S. official said it was taking as long as 12 hours for immigration officers at Al Udeid Air Base outside Doha, Qatar, to check arriving Afghans against the National Counterterrorism Center’s watch list. The official said that vetting and screening processes needed to move faster to prevent the evacuee pipeline from clogging up again at Al Udeid, the largest base receiving Afghans, as it did for several hours last week…

For the military, part of the problem is the sheer scale of moving so many people so quickly, with so little advanced notice. For instance, the C-17 military planes, which are transporting 400 people per load, have one or two bathrooms on them, and the flight from Kabul to Qatar is four hours.