Who promised what to whom? The American taxpayers and draftees never promised anyone anything. And in any case, if someone promises to murder innocent civilians, they are morally obligated to break, not fulfill, that promise.
To what end is this strategizing directed? If the goal is to protect freedom, security, and prosperity, then certainly there are better methods than murdering innocent civilians. More likely, however, it is simply one criminal gang trying to subjugate others.
First of all, there are at-large criminals everywhere, but it is never suggested that we should indiscriminately kill everyone in a certain area just because we know that there are some criminals among them. (Anyone who wanted to do this would rightly be regarded as a sociopath.) That’s why we have detectives: to find and target individual criminals.
To some extent, those who actively hide and shelter criminals are not entirely blameless, but no one is guilty of a crime simply for being a criminal’s neighbor. So when you ask, “How is justice done?”, what about justice for the innocent bystanders? Surely they have a better claim on their lives than someone who wants to get revenge on their neighbors?
And in a guerilla war, Al-queda can probably win in the long run. Winning in Afghanistan (assuming that it is possible) will not win the war against Al-queda: it will win a prolonged battle that is part of that war. Winning in Afghanistan, in the fashion that the United States sees victory, is also very questionable. I think there is a lack of understanding of Afghan history.
In the case of the Iraq War, the most valuable US allies have already left (apart from the UK, which will leave). The rest, I’m sure, are just waiting for the word.
The problem is that a large portion of South Vietnamese sympathisized with the Communists, because the totalitarian government backed by the U.S. in the south was even worse.
Why is everyone forgetting the Soviet involvement in Vietnam? Soviets trained Vietnamese, sent supplies and weapons and even their own military personnel to assist Vietnamese (maybe not on the same scale as US, but still significant). So, it was essentially an internal struggle with parties backed by the two superpowers which turned it into another arena for the not so cold Cold War. If it wasn’t for ideological difference, it might have gone unnoticed and the Vietnamese could have a chance to sort it out between themselves.
Now, was it moral to help the South Vietnamese in light of mass murder that happened once the Communists took control of the country? Only if it was done by by privately funded volunteers (think about some Brits fighting with Greeks against the Turks even when Turkey was still an ally of the Empire). Stealing from people to fund the war that doesn’t even affect them, drafting people against their will and committing various atrocities to the people you supposedly want to save - i.e. fighting evil with more evil - is definitely not moral.