The U.S. government now says it has little doubt that the regime of Syrian President Bashar al-Assad used chemical weapons against its own people on a “massive scale.” This makes a military response imperative but not necessarily urgent.

If the purpose of a retaliatory strike on Syria is to uphold an established international norm against the use of chemical weapons -- a purpose we strongly support -- then the best thing for the Barack Obama administration to do is to make its case for intervention as deliberately and publicly as possible.

Carried out at the United Nations and in foreign ministries, legislatures and public forums around the world, such a campaign can rally support for a prohibition that underpins security and peace, with positive ramifications far beyond the Middle East. It can also help isolate those who have defended and backed Assad, beginning with Iran, Russia and China, and undermine their ability to do so in the future. Although the Obama administration has rightly made a distinction between an action to uphold a global norm and one to change the course of Syria’s civil war, pursuing the former will also advance the latter.

A crucial element will be the evidence that the U.S. uses to support its case. As defense analyst Anthony Cordesman has noted, the forthcoming unclassified intelligence report on Syria’s culpability will be the intelligence community’s “most important single document in a decade.” Given the ugly blot on U.S. intelligence from Iraq, we hope that this report errs on the side of disclosing sources and methods.

Letting UN inspectors in Syria finish their work -- which they have said will take four more days -- is also essential to garnering global support. So, too, is allowing U.S. allies such as the U.K. and France the time for their own public debates, which are just starting. More time and public evidence might also enable the U.S. to persuade the Arab League to move from condemning Assad to endorsing action against him.

More broadly, we would hope that before any operation gets under way, Syria’s transgressions are given a prominent place on the agenda of next week’s Group of 20 meetings, and that it produces a debate that shames Russia’s Vladimir Putin for arming Assad and, so far, blocking effective action against Syria at the UN Security Council.

Russia and China will doubtless keep vetoing Security Council resolutions that justify a use of force against Syria. Yet there is substantial precedent in international law for taking action nonetheless, especially if the U.S. succeeds in attracting wide support. And documenting and publicizing Assad’s brutality will raise the reputational cost of defending him, a penalty that China may soon grow unwilling to pay: Unlike Russia, it has no naval bases to lose and needs Middle Eastern oil.

The more convincingly the case against Assad is made, the more likely he will be held to account for what Secretary of State John Kerry has called a “moral obscenity.” In that respect, a speedy military response is likely to be a less effective one. We have no expectation that a limited military strike will be of huge strategic value: The civil war will grind on, in part because the opposition has yet to offer up any clear alternative leadership. But in defending what should be one of the international system’s bedrock values, the U.S. and its allies will have achieved a victory.

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