As I’ve noted before, there are two models for American policy towards Afghanistan. In the Korean model, we identify Afghanistan as being uniquely important and dangerous as a result of its history, topography, and location, and commit to keeping troops there indefinitely. The Vietnam model, of course, directs us to get our troops out regardless of the consequences.
Trump clearly prefers the Vietnam model. He’s been trying to withdraw completely for most of his term, but has been frustrated by the military leadership and his own fear that the Taliban would parade into Kabul on his watch, which would make him a loser. It is very possible that his latest flurry of firings in the Pentagon was designed to assist him in the process of forcing a withdrawal. Biden basically agrees that, in today’s world, there is no justification for treating Afghanistan differently than a host of other countries that, deliberately or not, provide safe havens for terrorists throughout the world, but it is less likely that he would withdraw completely. His goal appears to be to maintain a small counterterrorism force there and to leave the country to its own devices in most other respects.
In short, the differences between the two on Afghanistan are a matter of degree, not kind.