Vietnam was essentially a proxy war with Communist China and the USSR/Russia, except that we ended up doing more than just supporting South Vietnam.
Who knows... maybe going "total war" on the place would have been a fast track to WW3.... Now that I think about it, the advent of nuclear weapons redefined what "total war" would mean. There would be no rebuilding a radioactive wasteland, after all.
Basically the same situation with the latest Middle East wars... Maybe the point is to push our power/interests just enough to get our way but not so far that humanity goes extinct.