Choice versus Necessity

Dr. Andrew Bacevich has been working overtime lately, with yesterday's Los Angeles Times op-ed and an "appreciation of Graham Greene" piece in the new issue of World Affairs.  The Graham Greene article is Bacevich at his best and most biting, but I was thinking about a lot about his op-ed.  I don't have time to address everything there (like the questionable historic implication that the British would have been better off abandoning World War I) but wanted to raise one point that's bothered me for a long time.  Bacevich recommends:

"...forget the Bush Doctrine of preventive war: no more wars of choice; henceforth only wars of necessity. The United States will use force only as a last resort and even then only when genuinely vital interests are at stake."

So how do you tell a "war of choice" from a "war of necessity?"  That's entirely dependent on your definition of "last resort" and "genuinely vital interests," and I think there's a legitimate debate to be had on both.  Let's not forget that most Americans probably would have called the Afghan war a "necessity" not long ago, even though we might have continued to try negotiating for the Taliban to hand over bin Laden.  Even in the seemingly clear-cut case of the invasion of Iraq, there was at least a substantial portion of Americans who would have called that war a "necessity" too.  Go further back to the 1991 Gulf War, which is commonly thought of now as a clear-cut war of necessity (Richard Haass has just written an entire book about this), and you'll find that many people considered it a bad war of choice at the time, arguing that we needed to give sanctions and diplomacy more time to work.  I've never found the "war of necessity/war of choice" dichotomy useful, because the relative prioritization of "vital interests" is always subject to change depending on who is defining them and one can argue either way whether we've really reached the "last resort" at any given time.  The difference may be clear in hindsight but rarely so when decisions are actually being made.  That's not meant as a justification for any war, just a suggestion that maintaining a strict Powell Doctrine-style security strategy is not as simple as I think Bacevich makes it out to be.