Desperate times call for desperate conflict-management measures. This weekend, at talks on Syria convened in Vienna at the behest of Russia and the U.S., diplomats called for Damascus and mainstream opposition groups to agree to a national cease-fire, in parallel with continued offensives against the self-declared Islamic State and al-Qaida-affiliated fighters. The five permanent members of the United Nations Security Council pledged to back a “U.N. endorsed ceasefire monitoring mission in those parts of the country where monitors would not come under threat of attacks from terrorists.”

Will this be a case of “the third time’s the charm” for peacekeeping in Syria? Early in the civil war, two international observer missions tried to stem the country’s collapse into all-out war. The first, sent by the Arab League in December 2011, was grossly incompetent and barely lasted a month. The second, launched by the U.N. in April 2012, was more professional but no more successful. The blue berets deployed to oversee a cease-fire that was collapsing as they arrived. They made strenuous efforts to monitor the rising violence, but China and Russia blocked any serious reaction by the Security Council. ...