FOR those who back muscular humanitarian intervention, both the words and deeds of Colonel Muammar Qaddafi provided absolute moral clarity. “Come out of your homes, attack [the opposition] in their dens,” he told his supporters on February 22nd. He called the protesters “cockroaches” and “rats” who did not deserve to live: language chillingly reminiscent of the broadcasts of Radio Mille Collines, which spurred on the perpetrators of Rwanda's genocide in 1994.

As he spoke, his forces had set their sights on Benghazi, their adversaries' stronghold. According to Human Rights Watch, a New York-based group, government forces had already killed 233 people in the preceding week. A bloodbath beckoned, in a city of 700,000 people. The United Nations Security Council invoked a fateful formula, urging the regime to meet its “responsibility to protect” its people. On March 17th the council, “expressing its determination to ensure the protection of civilians”, ordered air strikes.

That set the stage for the first full-blown test of a principle that the UN adopted in 2005 and has been refining since. The doctrine of “responsibility to protect” (R2P) holds that when a sovereign state fails to prevent atrocities, foreign governments may intervene to stop them. Human-rights advocates say it saves lives. Sceptics see it as too easily misused to be useful: a cover for imperialism, or even an incentive to kill (because even if a massacre is not looming, an unscrupulous warlord might be tempted to engineer one against his own people to spur outside support).

Previous uses of R2P have been solo ventures. In 2008 Russia used it to justify attacking Georgia, and France cited it after the cyclone in Myanmar, implying that humanitarian aid might have to be brought in by force if the regime persisted in stonewalling (it backed down). But before this year, no mission had been authorised by the UN Security Council that so explicitly cited the new principle.

Swamps and fogs

At first it looked likely that the doctrine would either triumph or die in Libya. But two months and thousands of air strikes later, war's messy reality has merely hardened views on both sides. On one hand, the decision to go to war was made in good faith at a time when the risk of massacres seemed real. As Mats Berdal, a professor at King's College London, points out, the world's leading powers had good reason to think they were “avoiding a Srebrenica”—the massacre of Bosnians which UN forces failed to avert in July 1995.

But as the war drags on and NATO strikes more widely, sceptics also feel their case has been bolstered. “For those of us who feared that R2P was just a warrant for war, our fears have been vindicated,” says David Rieff, an advocate-turned-critic.

Responsibility to protect gained ground after ghastly mass killings in the late 20th century, including massacres by the Khmers Rouges in Cambodia in the 1970s; the use of chemical weapons in Iraq in 1988; and ethnic cleansing in Bosnia. In 1999 NATO unleashed an air war, without a UN blessing, to stop a Serbian campaign in the province of Kosovo. It argued that the need to protect civilians was an overwhelming moral imperative. The UN gave a sort of retrospective blessing by endorsing an international tutelage for the territory, led by Bernard Kouchner, a French pioneer of humanitarian intervention.

But the terrible civil war in Iraq that followed America's invasion in 2003—portrayed as intervention against tyranny—shrivelled support for the doctrine. A possible result of that may have been hesitancy in intervening to stop the Sudanese government's genocide in Darfur.

Seeking to restore liberal hawkishness's good name, a group led by Gareth Evans, a former Australian foreign minister, pushed the UN's 60th anniversary conference in 2005 to endorse the idea that the world has a “responsibility to protect” civilians. Eventually 150-plus countries agreed to allow armed intervention through the Security Council “should peaceful means be inadequate and national authorities are manifestly failing to protect their populations from genocide, war crimes, ethnic cleansing and crimes against humanity.”

So much for the theory. What about the practice? Colonel Qaddafi provided an all-but-unique test. Regional leaders loathed him and readily dumped him. The Arab League's support for the intervention stopped Russia and China wielding their vetoes. And the concentration of the rebels in the east, combined with flat desert terrain, at first made the regime's forces easy bombing targets. “The stars were well and truly aligned in the Libya case,” says Mr Evans. “All the criteria were satisfied.”

The immediate goal of protecting Benghazi from massacre was achieved within days. Having destroyed Libya's air defences, Western bombers and missiles pummelled the advancing troops into a speedy retreat.

Harder decisions followed. Libya's army continued to shell other rebel-held cities, and its snipers were plainly targeting civilians. Protecting all Libyans, not just those in the east, would require the end of Colonel Qaddafi's rule—an outcome that both Western and Arab governments had already called for. NATO stepped up its military campaign, bombing retreating columns as well as advancing ones, and attacking command-and-control centres frequented by Colonel Qaddafi and his family. On April 30th an air strike killed one of his sons. The line between curbing atrocities and an air war for regime change blurred—though a land operation is ruled out, for the moment.

Your war is my argument

Both sides of the debate will eagerly cite Libya the next time mass murder seems imminent. It shows that a modest dose of air power can save lives; but also that the rhetoric of civilian protection can be stretched to justify a creeping mission. Power politics decides which lives get saved, and which policy aims triumph.

Mr Rieff decries a “two-tiered system of interveners and intervened upon”, where the “old imperial powers” make the rules. But which powers exactly? The Libyan vote passed only because non-Western Russia and China withheld their Security Council vetoes: all but unimaginable until recently. Both countries are now getting cold feet, claiming misuse of the resolution's elastic language. For different reasons Mr Evans bemoans excess zeal too: he wants to preserve the purity of R2P, and fears an interpretation that allows for “all-out aggressive war”. A lot rides on this war—and not just for the Libyans.