This phrase is getting kicked around a lot lately, I’m sort of in search of an operational definition. Fortunately, here’s Richard Haas:
Wars of necessity must meet two tests. They involve, first, vital national interests and, second, a lack of viable alternatives to the use of military force to protect those interests. World War II was a war of necessity, as were the Korean War and the Persian Gulf war.
If that’s what people mean by “war of necessity” then I think we can probably do without the phrase. “Necessary” is a very strong claim and the phrase, defined Haas-style, seems like a way to try to smuggle more heft than the situation actually warrants.
I’ll happily grant Korea as a great example of a “good war.” You have a country friendly to the United States becoming the victim of unprovoked aggression from an unfriendly country. Pretty much everyone believes that South Korea has a right to fight back in its own defense. And it’s only a very small leap from a belief in self-defense to a belief in the idea of “collective self-defense” whereby countries who are friendly to South Korea should help it out in its hour of need. That’s how Harry Truman saw it and that’s how I see it decades later.
Nevertheless, the Korean War doesn’t fit any intuitive concept of a given course of action being “necessary” for the United States. It’s not like once the DPRK’s tanks hit Seoul they’re just a hop, skip, and a jump away from San Francisco. “Necessary” implies that failure to fight would lead to national extinction. Of course the Korean War was a necessary war for South Korea. But the wisdom and morality of American involvement in the war is basically parasitic on that fact. It’s like mounting a “defense of others” argument in a criminal case. For the United States, which is conveniently located on the North American continent adjacent to two friendly and relatively weak countries, it’s going to be very hard for anything to meet a strict necessity test.