by the time Biden took office the US only had a few thousand troops left in country, what was holding the tide back was not force but the threat of force, the peace deal destroyed that and so the damn broke, to reverse it would have required sending back in tens of thousands or troops, probably a greater number than had ever been deployed
By all accounts the real failure was giving up Bagram Airforce base and trying to evacuate out of Kabul which was far less secure and so caused the operation to be rushed
|