Kofi Annan’s new mission to Syria is among the toughest of his diplomatic career. Assad is no pushover and is not hesitant to use force to maintain his position in power—as his brutal assault on Homs shows. The stakes couldn’t be higher, given the links to wider regional conflict and the heavy geopolitical overlay (read: Iran.) Most of all, for a U.N. mediator, strength comes from their international backing—and in this case, the U.N. Security Council has been bitterly divided. Yes, the Security Council recently issued a unified statement on humanitarian access, but that’s the minimum possible threshold of “action.”

To this tough test, Annan brings a number of advantages. The most important is that he has strong relationships with every permanent member of the Security Council, including the Russians and the Chinese. Not many diplomats can claim close ties to Russian Foreign Minister Sergei Lavrov and strong support from the United States; both will be critical if Annan’s mission is to succeed. On blogs and social media sites, Annan’s appointment was criticized because of his role in Rwanda, but in foreign ministries around the world his appointment was immediately hailed. Annan will take with him to Damascus support from virtually every capital that matters.

This puts Assad in a bind. My colleague Richard Gowan wrote of the previous Arab League mission that even though observer missions can’t stop violence, they can narrow the diplomatic field by making it evident who’s at fault and what’s at stake. The Arab League mission did do that to some extent, but it had dreadful leadership and a flawed structure, and so didn’t quite back Bashar al-Assad into a corner. Annan, with his experience and prestige, but more important with broad international backing and good relations with Russia, leaves Assad much less room to maneuver. If Annan’s mission fails, and the odds of that are still fairly high, the diplomatic option will be well and truly exhausted.

Annan’s mission takes place against the backdrop of a debate that has hinged on unrealistic and risky options: military intervention (by who? In what timescale? With what death toll?), or arming the rebellion (with what endgame, other than rapid escalation of the civil war? With what tools to stop Syria becoming a proxy battleground for the region?). Can the diplomatic option still succeed?

Much hinges on Russia. Prime Minister Putin’s statement on March 2 distancing himself from Assad’s regime is a glimmer of light. If Russia is willing to shift toward a unified Security Council position on a transition, based on the Arab League plan, much is possible. Among the options then is a set of political transition arrangements, and even “negotiated consent” for a stabilization mission, perhaps similar in structure to the hybrid U.N./multinational force now deployed in Southern Lebanon. Such a force could be deployed if the Syrian armed forces are ordered back to barracks and a ceasefire called. That Putin talked of a ceasefire is a useful opening for Annan.

Russia and China have every reason to back Annan, because a U.N.-led solution gives them a seat at the table the way a U.S./NATO or Qatari/Saudi driven outcome does not. The U.S. has reasons to go along with that arrangement. Although Assad’s alliance to Russia is a part of the geopolitical chessboard the U.S. would like to see shift here, that alliance will be diminished under any circumstance, and an outcome that Russia can live with is probably also one that avoids the U.S. getting entangled in intervention in yet another Middle East crisis. A less than ideal outcome for the U.S. will come at vastly less cost and risk than a theoretically purer result.

U.N. mediators are dealt a weak hand: the secretary-generalship itself is a position with many vulnerabilities and few strengths. When U.N. mediators succeed, it’s partially down to personal political skill, but mostly it’s the coalition they can assemble behind them. Secretary-General Ban Ki-moon showed a deft and confident hand in selecting his predecessor to play that role in Syria. If Russia and China are looking for a way out, Annan just might give it to them.