The U.S. got in line today with its European allies on the question of Syria and chemical weapons: It now says it has evidence that Syria used sarin gas, but that this evidence needs to be confirmed before devising a U.S. response.

Only a day earlier, Secretary of Defense Chuck Hagel had deflected reports of an Israeli general’s blunt assertions that Syria had used chemical weapons. “Suspicions are one thing. Evidence is another,” Hagel said. We are skeptical that new evidence suddenly came to light in the past 24 hours, and have our own suspicions -- namely, that President Barack Obama was intent on blurring the one red line that he has said would trigger “consequences” for the regime of Bashar al-Assad.

Even so, we agree with the White House that a higher standard of proof is required to launch a military response. “Given the stakes involved, and what we have learned from our own recent experience,” the White House wrote in a letter to senators, “intelligence assessments alone are not sufficient -- only credible and corroborated facts that provide us with some degree of certainty will guide our decision-making.”

Yes, we all remember Iraq’s missing weapons of mass destruction.

So what will it take to achieve that clear proof? A United Nations investigation has been stalled by Syria’s insistence on limiting its scope to a March clash in which government and rebel fighters have accused each other of chemical strikes. The UN wants to explore other reports as well.

The U.S. and its allies should press Russia, which has the same interest as other world powers in containing the chemical threat, to persuade its ally Assad to give the UN team a full mandate.

In the meantime, the U.S. should work with rebels and refugees to collect samples of soil and foliage, as well as hair and urine from people in areas said to have been hit. It was France and the U.K. that tested samples and took the results to the UN to demand a full investigation. The U.S. can’t afford to take a back seat again.

Here’s what we do know: Syria has one of the world’s largest stockpiles of chemical weapons, including mustard gas, which is a blistering agent, and sarin and VX, both nerve gases. The use of these weapons has been prohibited since the signing of the Geneva Protocol in 1925; the Chemical Weapons Convention in 1997 committed its signers, including the U.S. and Russia, to the verified destruction of their chemical arsenals. As of February, signatories had destroyed 76 percent of their chemical weapons. Syria is one of eight nations that haven’t signed the treaty.

If the Syrian regime begins to distribute weapons for use against the opposition, it may also be emboldened to pass some to its Hezbollah allies in Lebanon, presenting a clear threat to Israel. If Assad begins to lose control, chemical weapons may fall into the hands of al-Qaeda-linked fighters, posing a global threat.

The Pentagon estimates that at least 75,000 troops would be needed to secure Syria’s chemical weapons if the country came apart. Such daunting numbers must not be allowed to block action if proof of chemical weapons use emerges. Instead, a graduated range of responses to Syria’s chemical threat seems both feasible and desirable.

Any tactical use of weapons should draw a response. That could mean strikes by cruise missiles tipped with high-temperature warheads, to destroy a known chemical weapons depot, parked Syrian jets or any military target. The nature of the target is less important than the clarity of the message sent.

The U.S. and its allies should also condition more help for the Free Syrian Army on its commitment to secure and later destroy these stocks. A new Syrian government should sign the Chemical Weapons Convention.

Army General Martin Dempsey, chairman of the Joint Chiefs of Staff, recently stressed that any military intervention in Syria must have a clear, achievable outcome, and that none was obvious. Securing chemical weapons, if that becomes necessary, would be such a goal.

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