The United Nations Security Council passes a resolution noting “widespread and systematic attacks” on civilians and authorizing U.N. member states “to take all necessary measures … to protect” them. It invokes a little-known concept called the responsibility to protect (or R2P) as justification for the use of force to stop the slaughter of civilians.

Against which nation did the Security Council take this action? No, it wasn’t Syria. It was Libya. U.N. Security Council Resolution 1973, approved on March 17, 2011, authorized the use of force against Libya “to protect the Libyan population.” That was the sole justification for the intervention, according to the Security Council.

So why is there a so-called “responsibility to protect” Libyans but not Syrians? After all, the death toll in Syria - 8,000 to 9,000 - far exceeds what was happening in Libya; yet the Security Council has not authorized an intervention. You could argue that a Syrian intervention at least could actually stop the ongoing massacres, as opposed to stopping one that might occur - the justification for the council taking action against Libya.

There are obvious reasons why the Obama administration advocated the use of force in Libya but has not in Syria. It was tactically and militarily easier to go into Libya. The Russians and Chinese are more dug in against using force in Syria. Many Arab states welcomed Moammar Gadhafi’s downfall but fear the destabilizing effects that overthrowing Bashar Assad might have on Syria’s neighbors, including Iraq and Lebanon. And frankly, Europeans, particularly the French, saw more economic advantages from an intervention in Libya than they see in Syria.

The use of force in Syria may indeed be impractical. But invoking R2P to justify intervention in Libya but not in Syria is hypocritical. Surely the lives of civilians in Syria are as worth saving as the lives of civilians in Libya. The R2P doctrine is presented as a universal moral imperative and a guiding principle on when to use force. If it is a moral obligation of the international community, R2P supporters would be equally obligated to advocate the use of force when it is the best and only means to achieve their ends.

The mistake R2P supporters make is using the idea to justify military intervention. R2P never should have been invoked in Libya. If the idea is, as advocate Gareth Evans argues, mainly to “generate a reflexive response” from the international community, it should never be deployed as a moral argument to obligate U.N. member states to support an armed intervention. It’s one thing to claim that all nation-states are obligated to protect their citizens. It’s another to argue that the international community is obligated to support an armed intervention. That should be reserved for self-defense or U.N. Charter Chapter VII interventions authorized by the Security Council to ensure international peace and stability.

Some R2P supporters recognize the dilemma caused by the Libyan intervention. Lloyd Axworthy, a former Canadian foreign minister, contends that R2P “should not be judged on the basis of the military response in Libya.” Mr. Evans argues that circumstances should determine whether force, sanctions or some other measure is used.

But it’s those circumstances that dilute the moral power of R2P as a justification for using force. Again, if force was a “last resort” for Libya, why not for Syria? If circumstances can exempt U.N. member states from undertaking the only action that can truly stop the killing, what does that say about the nature of the obligation?

The responsibility-to-protect idea is fine so long as it applies to how sovereign governments should treat their people. But it fails as a guide for when the international community should support an armed intervention.

Kim R. Holmes, a former assistant secretary of state, is a vice president at the Heritage Foundation.