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.
Matt argues that the Korean war — though “good” — seriously stretches the limits of “necessary,” and because of the extremely limited case of wars that genuinely do hew both conditions, the concept of “war of necessity” should be mostly discarded as a framework for evaluation US commitments. I tend to agree, but I think the problem is actually more with “vital national interests,” than the word “necessary.” After all, the reason the Koren war shouldn’t be judged a necessary one is because it’s difficult to argue that the defense of South Korea from an aggressive neighbor was vital to the national interests of the United States. Certainly, inasmuch as it preserved a favorable status quo, the defense of South Korea lay within the interests of the United States, but it’s a bit of a jump to go from here to its “vital” importance. As such, I really don’t think there’s any problem with the framework itself, we just need to be willing to concede that a good portion of our military misadventures are not in fact necessary. And it follows then that an unnecessary war should be subject to strict consideration of its costs and benefits. A perfect example here is Afghanistan, where I think the case that denying al Qaeda “safe havens” is vital to our national interests is at best unclear, and accordingly we should get a better sense of the possible trade offs to our presence.