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The withdrawal didn't cause the Afghan army to fail. They've been failing for years before that. If they couldn't get it together over the last 20yrs no amount of time was ever going to fix it.
For one they left the Afghan army strung out trying to defend isolated outposts, configured to US interests still - even just taking the time to transition the Afghan army into a configuration for their own interests would have at least given them a fighting chance - never mind all the messing about with the situation with their air-force, which largely seems to have been driven by politics and making money - leaving them with a fast depleting stock of Russian helicopters and a messy situation trying to adapt to US helicopters which were delayed in delivery any way.
The withdrawal didn't cause the Afghan army to fail. They've been failing for years before that. If they couldn't get it together over the last 20yrs no amount of time was ever going to fix it.