Rory Stewart says humanitarian intervention is like mountain rescue—protecting lives doesn't require destructive adventures.

And so, to return to the question about Libya and Syria: This may sound awfully cold, but we're bombing Libya because we can and because it might have good effects; and we're not bombing Syria because we can't, and it almost certainly won't.

Libya is, in fact, the most straightforward case for "R2P" action that's come along in years, maybe decades. (The widespread claim, repeated in n+1, that Samantha Power, now a member of the National Security Council, persuaded President Obama to intervene is overstated.) Muammar Qaddafi was crushing a popular resistance; he said publicly that he would soon send his hired thugs door-to-door to exterminate the protesters, tens of thousands of them, like "rats." He had the power, and seemingly the will, to make good on his promise. So the Arab League unanimously passed a resolution (a nearly unprecedented event in itself), pleading for the international community to take action. The U.N. Security Council followed with a similar resolution, which neither Russia nor China vetoed. If the Western leaders hadn't responded under these circumstances, they may as well have announced that "humanitarian intervention" as a concept was dead.

Taking action was also a good idea from a realpolitik angle. The controversy unfolded in the wake of the Arab Spring; it was in our interest for the United States and NATO to appear on the side of a popular uprising against a quasi-allied dictator (and Libya's was about as quasi an ally as could be imagined).

While many criticized Obama and NATO for doing too little, too late, I suspect that, in the end (which now seems imminent), the effort will seem about right: assisting the rebels with air support (and probably more "training and equipping" by special-operations forces than is acknowledged) but not taking the lead—and, therefore, not getting lassoed with responsibility for determining, or fully funding, the new Libyan order afterward.

It's an approach that the authors of Can Intervention Work? probably appreciate. Stewart, to the extent he supports intervention at all, advocates an approach he calls "passionate moderation," while Knaus calls his attitude "principled incrementalism."

Stewart is a well-known author, and currently a member of the British Parliament, who was once more enthusiastic about these sorts of ventures, until he saw intentions run awry and good sense run aground in Iraq. (Until recently, he also ran an NGO in Afghanistan.) Knaus, Stewart's colleague when they both recently taught at Harvard's Kennedy School of Government, spent many years working with human-rights groups in the Balkans.

Stewart's half of the book is about why intervention in Afghanistan is failing, Knaus' half is about why intervention in Bosnia and Kosovo succeeded.

Both authors believe that, while there are some "crises that 'the International Community' cannot address," there are others that it can, and in those instances, there are some well-worn guidelines to follow—the main one being: There are no universal guidelines. Taking another useful metaphor from mountain rescue, it's best to learn the environment, be honest about your capabilities, and trust an experienced guide.

The main problem in Afghanistan, to Stewart, was precisely the international community's tendency to apply abstract concepts—like "governance," "stable state," and "rule of law"—to a country where they had no resonance and where the members of the international community possessed little or no knowledge of the languages, the culture, or the traditional social structures. He notes, for instance, that a 2004 document titled Securing Afghanistan's Future, written with the assistance of 100 international experts, and containing 137 pages, with 69 tables and charts, does not include any of the following words: Pashtun, Tajik, Islam, Shiite, jihad, Northern Alliance,or insurgency.

Both authors leave occasional holes in their arguments. Stewart, for instance, writes that, a few years ago, he actively opposed increasing U.S. troops, advocating instead a "light long-term footprint," mainly of aid workers, but he doesn't explain how he would have supplied them with security. Knaus, in examining various theories on why Bosnia worked, dismisses but never really refutes those that give much credit to the preponderance of U.S. military strength.