There is no such thing as a just war. All wars are unjust, including the world wars. Some wars are a necessity to combat significantly greater evil, again both world wars. The American war in Vietnam, which was not restricted to Vietnam, but included northern Thailand, Cambodia, Laos and wherever the joint command felt bombs should be dropped or covert activities were needed, offers no justification beyond greed and paranoia, i.e. the domino theory.
Chicken hawks are quick to point out justifications of thwarting communist world domination, and associated dictatorial governments, yet such economic systems have repeatedly failed without US military confrontations. The only truth is that those who favor wars like this one have no faith in the American people and their choice of a capitalist economic system. The Soviets are history. China is on a path returning to it capitalism of 5,000 years or more, while the communist party of China has 90 million members out of a population of 1.393 billion, it too functions more as holding corporation of other businesses than a true communist economy.
Vietnam was a French mess, left over from the post WWII attempt by the French to keep its colonies in SE Asia. The French failed and the US went in to clean up the mess, a police action. The reality was that we were fighting a nationalist movement, an anti-colonialist movement based on our own revolution for self rule. The hypocrisy of sacrificing 50 thousand young Americans dead for such cause, untold others emotionally destroyed, the establishment of a cultural schism in this nation, is mind boggling and certainly unjust.
I have no doubt there will be those who will say the war was confined to Vietnam, except when we went after Viet Minh supply routes in neighboring countries. I personally went on patrols in Laos and Cambodia, the targets were not Viet Minh. The targets were organized crime, warlords who sold drugs and traded weapons of both nations with American and French expats as well as American and French organized crime parties buying drugs from those warlords. Had we not had a military presence in the region, there would have been little or no drug trade to bring SE Asian opiates into the US and Europe. And let's not fool ourselves, had not British colonialists from India not introduced those opiate crops from northern India, todays Afghanistan and Pakistan, to SE Asia so as to maintain alternate sources, the problem would not have existed at all. Greed, the American way, in the end. Our true raison d'être. American blood for greed, money.