Vietnam is a bit more complex. The war started under Eisenhower, with promises of unlimited support made by VP Nixon. In many ways, Nixon was the alpha and the omega of the Vietnam War. But both Kennedy and Johnson went along with those promises, even though with 20/20 hindsight they should not have. It was their decision.
Since 1993, Democratic military intervention has been "lead from behind", with some air support, or even special forces support. It has avoided almost all American casualties, but has meant we have less say in what follows the war.
Bush was more interested in the dozen invasions of the Neo-Cons. Each invasion was supposed to setup a "seedling democracy" in the region, and also fund the next invasion. The Neo-Cons, and more particularly the Vulcans, thought that if they picked the right countries, they could flash nation build them in a few months. Afghanistan was not one of the right countries, but supposedly Iraq was.
trump was all over the place in foreign policy. The Tongo Tongo Ambush is instructive on this. trump sent troops to Niger to fight ISIS, maybe local rebels, or who knows who. They had no air support, because trump was sure he could convince someone else to provide air support if they were attacked. When they were attacked, trump was too busy playing golf to try to get air support. A couple of months later he called one of the widows and said, "he[the dead soldier] knew what he signed up for, but it hurts anyways..."