Morale collapse is a real thing. There are many recent cases.
The fall of Mosul to ISIS in 2014 (and about half of the Shiite parts of northern Iraq) went down exactly the same way.
The ISIS offensives in Eastern Syria in 2014 in fact also went the same way, as did the attack on Palmyra in 2015.
I thought the 900 US troops were there since the former Trump administration, and remember when Trump had the Iran military guy killed, many thought that was a good thing. I don’t think Biden made any increases from the number Trump left there. These 900 were specifically to block ISIS, and to help the Kurds who gave us lot of support in northern Iraq in earlier wrs. Unfortunately, there are Kurds in conflict in Turkey with the Erdogon government which is a complication there.