public opinion
The public votes for the congress, and to try to keep their position, they would usually go with the larger crowd. In the case of the Vietnam war, many had decided that a war in a far away 3rd world country wouldn't affect the US, and that it would be a waste of time, money, and effort. Because of the shift in the public opinion from supporting the war to wishing the war would end soon, the US pulled out their troops and allowed the Communist to take over Saigon, and the rest of Vietnam.
Public decision have been playing a major role in many different times, for example:
World War 1: where the US thought of staying out of the war until the sinking of Lusitania and the zimmerman telegraph
World War 2: Bombing of Pearl Harbor
These are a few other examples of the crowd making the major decision and Congress going along with it
hope this helps
