Trump also ordered the Pentagon, against Mattis’s recommendation, to come up with a plan to withdraw approximately half of the American troops deployed to Afghanistan, a move that military officials have warned could plunge the nation into chaos.

via https://www.washingtonpost.com/world/national-security/trump-announces-mattis-will-leave-as-defense-secretary-at-the-end-of-february/2018/12/20/e1a846ee-e147-11e8-ab2c-b31dcd53ca6b_story.html

Mattis even resigned due to not being able to comply with Trump’s idiotic plans.

I guess it’s possible that Biden could have just scrapped the whole plan and do something else. But most of the damage was already done by then.