The US was driven by Cold War concerns about the spread of communism, particularly "domino theory" the idea that if one Asian nation fell to the leftist ideology, others would quickly follow.

BBC: NEWS

But those that pushed the domino theory were ultimately unable to prevent the erosion of public support for the Vietnam war and it may be that the time is already past for the theory's echo to steady those who doubt that any project conceived in the West will commend itself to a majority of Afghans.

ECONOMIST: Lessons for Afghanistan

There were many reasons he decided against withdrawal, including his political fears that advocating for such a policy would play directly into the hands of Republican critics such as Richard Nixon, and because he did have some sympathy for the "domino theory, " which stated that if South Vietnam fell to communism, neighboring countries would follow.

CNN: Commentary: Obama should listen to Biden