Syria chemical weapons attack
Animal carcasses lie on the ground, killed by what residents said was a chemical weapon attack on Tuesday, in Khan al-Assal area near the northern city of Aleppo, March 23, 2013. (Reuters/George Ourfalian)

TEL AVIV — The growing evidence that the Syrian regime has used chemical weapons against its own population provides an opportunity for the United States and Russia, the two countries that have the most influence over the situation, to jointly force President Bashar al-Assad to hand over power.

A senior Israeli military officer told several journalists here today that he’s “very close to 100 percent sure” that Assad’s regime used the nerve agent sarin on March 19, and has probably used chemical weapons on several other occasions since then. The Israeli account matches similar allegations made last week by Britain and France, but the U.S. so far has been quiet.

Assad’s use of the chemical weapons crosses the “red line” stated last month by President Obama. But perhaps more important is that it also violates the warnings made privately to Assad by Russia. The Russian caution towards Assad about chemical weapons was noted by Vitaly Naumkin, director of the Institute of Oriental Studies at the Russian Academy of Sciences and a Russian expert on the Middle East, in response to a question at a security conference here organized by the Institute for National Security Studies, an Israeli think tank.

The crisis should convince Russian President Vladimir Putin that it’s time to abandon his support for Assad and begin to work for a political transition in Syria. Assad is apparently so desperate that he has ignored warning about the chemical weapons not just from Washington, but also from Moscow. Will Putin really allow the Syrian dictator to use weapons of mass destruction in defiance of Russia?

The shared U.S. and Russian opposition to any use of chemical weapons in Syria was discussed in February by Russian Foreign Minister Sergei Lavrov and Vice President Joe Biden when they met on the sidelines of a security conference in Munich. The two also talked about ways they could cooperate in safeguarding the weapons if Assad’s regime is toppled. Yet Russia said in March, after the initial reports of Syrian chemical weapons use, that both sides in the Syrian civil war had made claims and called for a UN investigation of all such accusations. Lavrov summarized this position Tuesday in comments to reporters in Brussels and said investigators should focus on specific reports about the weapons’ use, rather than make a broader inquiry.

Has the Obama administration pressed Moscow about Assad’s violation of their joint red line—and proposed that the offense is a basis for action to force Assad to hand over power? I hope so. That’s the only explanation for Washington’s strange silence as the evidence mounts of Assad’s brutal tactics.

With this latest evidence of Assad’s unlimited war against his own people, it’s time for Russia to join an international statement that the Syrian leader has gone too far—and must be replaced. This crisis is truly an opportunity, if leaders are bold enough to seize it.

David Ignatius writes a twice-a-week foreign affairs column and contributes to the PostPartisan blog.