It may take awhile to determine whether, as the opposition claims, Syrian President Bashar al-Assad used chemical weapons to slaughter more than 1,000 of his citizens. If he has, this brazen atrocity not only changes the U.S. calculus in the region but also requires a clear response from the civilized world.

Words and resolutions would not do for a massacre of this scale. Ideally the United Nations Security Council would sanction limited international military action to punish and deter Assad. If not, the U.S. should assemble the broadest possible international coalition to deliver a message, with force if necessary, to the regime.

The immediate priority is to get U.N. inspectors, already in Damascus to investigate previous allegations of chemical weapons use, to the site in nearby Ghouta. Getting access for inspectors may be difficult. Assad clearly doesn’t give a hoot what the rest of the world thinks, so the U.S. and its allies should try to persuade his backers, especially Russia, to apply pressure. President Vladimir Putin surely wouldn’t want to be stuck defending this atrocity and may need to be pressured publicly to cooperate (there seems to be little to lose in the U.S.-Russia relationship at the moment). In addition, members of the Security Council should be aware that chemical weapons use on this scale would set a precedent that none would want to endorse by default.

The reality remains, though, that the U.S. and its allies may (as in the past) have to make their own determinations of what weapons were used. If so, the administration should prepare for the worst: another affirmation that, by using chemical weapons, the Syrian regime crossed the “red line” articulated by President Barack Obama last year.

In a letter to Representative Eliot Engel written before the Ghouta attack, Martin Dempsey, the chairman of the Joint Chiefs of Staff, set out the reasons for administration’s reluctance to intervene in Syria. If the U.S. is to back a party in the conflict, that party “must be ready to promote their interests and ours when the balance shifts in their favor,” he wrote. “Today, they are not.”

Dempsey and the White House are right not to want to own another conflict in the Middle East. Yet failure to react also has repercussions, in Syria and beyond. Why should Iran, or indeed Egypt’s new military rulers, take U.S. commitments and red lines at face value? In addition, Syria looks set for years of continued civil war in which each side is supplied by regional backers, and spillover to Syria’s neighbors is inevitable. It can’t be in U.S. interests for this war to include chemical weapons.

Nor do Dempsey’s justified concerns about the nature of Syria’s opposition preclude action. The U.S. should accompany any response to a proven use of chemical weapons by Assad with a clear statement of its policy goal in Syria: not to topple the regime or ensure victory for part or all of the opposition, but to force the main parties to a cease-fire. There are ways to do this short of a full-scale U.S. intervention, and an internationally endorsed statement of these limited goals would help to guard against mission creep.

If the claims against Assad prove true and Russia vetoes action by the UN, the U.S. and its allies will need to present a defensible legal justification for acting outside the Security Council. That’s not impossible. Working with allies such as Jordan and Turkey that have been affected by attacks and incursions from Syria, the U.S. could build a consensus for action.

As Dempsey’s letter makes clear, the administration has chosen a noninterventionist policy in Syria. Almost exactly one year ago, Obama said that “a red line for us is we start seeing a whole bunch of chemical weapons moving around or being utilized. That would change my calculus.”

Today’s allegations, when figured into that calculus, may soon demand a more forceful response. Whether it is cruise missiles against Syria’s air force or another military option, it is a response the president ― and the rest of the world ― should be prepared to deliver.

(Bloomberg)