There are three militaries in this story.
The US, the Taliban, and the Afghan National Army (ANA).
The US and the Taliban have had a cease-fire for over a year. The Taliban granted that cease-fire because the US promised to withdraw.
The US was keeping Kabul secure, but also withdrawing. The ANA was supposed to hold Kabul for a month or two while the US withdrew. You can't maintain security and withdraw at the same time, obviously. Relying on the ANA to do anything was wrong, obviously. Perhaps agreeing to the cease fire and withdraw deal was also a bad play. But withdrawing without telling anyone, even if possible, would have been worse. So many more people would have been left behind.

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