The wars about which there is the least dissent, both contemporary and historical, are those which are judged to have been good, necessary, and just. And though there can be extensive debate against how much any war fits any or all of these categories, it’s hard to doubt that a war that is seen as good, just, and necessary is a “better” war than one that fit into none of those categories.
We can use Iraq as an example. Some would contend that America’s invasion of Iraq was none of the above. Not good, not necessary, not just. The vast consensus at the time, however, was that it was a good war, and if not a just war, at least necessitated by weapons of mass destruction.
Goodness in war is something judged by external moral absolutes. America’s mythical neoconservatives like to fight wars against evil. In such a black-and-white world, all wars waged by America are inherently good. Even if one doesn’t believe that America is always on the side of the good, there are some clear situation where we unquestionably wage wars on the side of the good. World War II, which is generally the most clear-cut war in history, saw the Allies fighting the good fight. It would be essentially impossible to define either the Nazis or the Japanese, both of whom believed they were racially superior and thus engaged in genocidal tactics, as much other than evil.
Necessity is perhaps more difficult to pin down than good. Realists, who believe in unwavering pragmatism in foreign policy, generally prefer to fight only the necessary wars. One can easily say that it is necessary to fight back when your territory has been invaded and your citizens are being killed. Leaving aside the Dalai Lama, who doubts the necessity of war for even self-defense, it’s generally acknowledged that a defensive war is a necessary war. More recently, it has also become recognized that in cases of genocide, war is necessary. It is with this belief that the Vietnamese invasion of Cambodia–ending the reign of the Khmer Rouge–was necessary, that United Nations intervention in Bosnia was necessitated, and NATO action in Kosovo was legitimate.
This, however, gets to the final and most difficult point. When is a war just? Some liberal institutionalists believe that a war is only just if it has the blessing of the biggest international body of all: the UN. In this view, only the intervention into Bosnia was just. Because NATO intervention into Kosovo didn’t come with United Nations assent that it was good and necessary, the war was unjust. Others would say that assent from any existing multilateral institutions can make a war just. Thus, intervention in Kosovo, because it was blessed by NATO, was more legitimate than intervention in Iraq, where assent only came from an ad-hoc “coalition of the willing.” As it was viewed at the time, Vietnam’s intervention into Cambodia was actually the least just of all of these; it was completely unilateral.
But most commentators now agree that Vietnam’s intervention in Cambodia was if not just, at least good and necessary, and thus worthy of respect. Rarely is a war waged by anyone seen by the whole world is good, necessary, and just. In this respect, WWII is a widely recognized exception.
It should also be noted that a war that seems good and necessary, if not just, when it begins is not necessary seen as such when it ends (or in historical hindsight). It’s hard to deny that America’s involvement in Vietnam, beginning with Eisenhower and not ending until the presidency of Gerald Ford, was initially seen as good and necessary. Good because Communism was broadly seen in America and the western world as irredeemably evil, necessary because without it all of Asia would fall to the evil of Communism. Yet today–and in some quarters, at the start of the war–it’s recognized that it was neither good nor necessary. The Vietnamese may have embraced communism, but are widely seen to have been seeking only independence. And the string of dominoes theory–if one falls the rest will too–is widely recognized as both unrealistic and silly.
Thus, in hindsight, Vietnam is seen as neither good nor necessary (it was never widely seen as just). It is thus widely seen as one of America’s lowest moments and worst wars. Wars that were not good, and not necessary, and not just are usually and understandably sources of national shame.
And though one could reasonably argue that all wars are a shame, it’s hard to deny that without at least goodness and necessity, or justness and goodness, or justness and necessity, a war truly is a shame.