Syria’s President Bashar al-Assad on Tuesday accepted a United Nations-backed plan to end the bloodshed he is inflicting on his country.

The plan is a lowest common denominator, repugnant to Syria’s opposition and one of several proposals Assad has accepted over the past year, only to ignore them. It’s also the best option available.

A good vantage point from which to gauge Syria’s options Tuesday was a hotel in a suburb of Istanbul, where Turkey and Qatar had corralled more than 300 members of Syria’s fractious opposition. The message they received was clear: Unite, because if you don’t, it’s hard to see how the outside world can do much to help you on April 1 when the Friends of Syria group meets to discuss next steps.

Uniting turned out to be tough. The delegates tried to agree on a National Covenant, it took hours longer than expected, and ethnic Kurdish delegates walked out, unwilling to sign a document that didn’t offer more explicit recognition, equality and protections to Kurds. Their message was clear: Yes, more than 8,000 people have been killed over the past year, but unless clear guarantees are in place for minorities, things could get a whole lot worse after the regime falls.

That is the heart of the problem in Syria: As appalling as Assad’s butchery has become, as badly as the world may want to help stop it and bring him to justice, there aren’t enough tools available right now to ensure that, in a country riven by sectarian and ethnic tensions, the outcome of more active intervention isn’t even worse. That goes for Syria itself, and for the wider region, so long as Assad retains the support of Russia and Iran.

We’ve argued before that arming the opposition would only escalate the conflict and casualty rate, with little prospect of defeating Assad’s military on the ground. So what about a Libya-style international intervention from the air, as U.S. Senator John McCain has urged?

This month, when the Pentagon’s top brass appeared before a hearing of the Senate Armed Forces Committee to talk about Syria, the answers from Secretary of Defense Leon Panetta and Chairman of the Joint Chiefs of Staff General Martin Dempsey made it all too clear why the U.S. military is reluctant to act.

First, unlike in Libya there is no United Nations Security Council resolution to support intervention. Second, Syria has a large stockpile of chemical and biological weapons that could disappear in a likely breakdown of order -- Dempsey assessed that risk at 100 times greater than in Libya. Third, Syria’s air defenses are five times more sophisticated than Libya’s and are concentrated in densely populated areas, making civilian casualties inevitable. Fourth, Syria’s military is better trained, better equipped and more disciplined than Muammar Qaddafi’s loyalists were. And fifth, Syria’s opposition -- an archipelago of 100 armed groups, with no central command -- is even less united than was Libya’s.

That leaves us with the plan drawn up by former UN Secretary General Kofi Annan, which critically if perhaps cynically has been accepted by Russia and China and, on Tuesday, by Assad. It calls for a cease-fire on both sides, withdrawal of heavy weapons by the government and access for news reporters, among other terms.

Success remains a long shot, given Assad’s appalling record and the gulf of mistrust that separates him from his opponents. The opposition members in Istanbul on Tuesday were deeply skeptical and disgusted by Assad’s tour of a sanitized Baba Amr, the suburb of Homs that Syria’s military cleared after a sustained shelling that caused as many as 100 civilian casualties a day.

The U.S. and its allies should do everything they can to support the Annan plan. But they should also prepare for the possibility of failure. President Barack Obama should quietly order up the detailed, exhaustive preparations for deployments that Panetta and Dempsey said would be the next step in making intervention an option. Those preparations could help focus Assad’s mind, and those of his supporters in Moscow, on the consequences of diplomatic failure.

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Today’s highlights: The editors on China’s clean-coal technology, Ireland’s economic woes and Syria’s muddled opposition. William Pesek on Japan’s workplace discrimination. Margaret Carlson on Trayvon Martin and Al Sharpton. Meghan O’Sullivan on Iraq’s economic salvation. Clive Crook on the writings of a potential World Bank president. Peter Orszag and Peter Diamond on Mitt Romney’s Social Security plan. And Simon Johnson and James Kwak on how the U.S. became banker to the world.

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