It shouldn’t have been this hard, but Secretary of State John Kerry has finally gotten Russia to back the peace plan on Syria that it endorsed in principle last June. This isn’t a breakthrough, but at least it’s a beginning.

What the United States and Russia seem to have realized is that a negotiated transition of power in Syria is better than a fight to the death, which would destabilize the region. That’s a wise judgment, but it’s not clear that it’s shared by either the Alawite clique backing President Bashar al-Assad or the Sunni jihadists who are the backbone of the opposition.

The formula, as expressed by one U.S. official, is that Assad will step aside “as part of a political process once a transitional governing body is formed.” The United States, in other words, is making Assad’s departure an outcome of the process, rather than a precondition. The Russians, in publicly backing the plan, are in effect pledging that they can deliver people in the Syrian government who would be part of a post-Assad transition.

Kerry described the common goal this way as he stood next to Sergei Lavrov, his Russian counterpart: “We’ve . . . affirmed our commitment to a negotiated settlement as the essential means of ending the bloodshed.”

But what will the United States and Russia do to implement this promise? Will the United States permit Iran to attend the international peace conference, as Russia likely will urge? The official U.S. position is that Iran shouldn’t attend. My guess is that President Obama would bend if he thought an Iranian role would create a more durable settlement of regional tensions.

Will the Russians lean hard on both Assad and the Iranians to force them to accept the reality that Assad is finished? “We are not interested in the fate of certain persons,” Lavrov said obliquely Tuesday. Lavrov spoke with Syrian Foreign Minister Walid al-Moualem, a pragmatic figure who might be a transitional player. But it’s doubtful Moualem, or anyone else acceptable to the opposition, could deliver the regime’s hard-liners.

This peace plan, like so many others for the Middle East, is a bet that moderates can carry the day. With tragic regularity, this hope has proved to be misplaced. To succeed this time, the United States and Russia will have to empower some regional or international force that can step between the combatants after the transition begins and minimize the killing.

A moderate rebel faction has finally begun to emerge behind Gen. Salim Idriss, the commander of the Supreme Military Council. He’s taking responsible positions — pulling back his forces from reprisal attacks after last week’s massacres of Sunnis in coastal villages. Idriss had also offered to negotiate with the regime, meet with the Russians, protect the Alawite community — and forswear chemical weapons.

The challenge for Idriss is to show that he can back these sensible positions with enough military muscle that his moderate forces, not the jihadists, hold the balance of power among the rebels. Idriss’s ability to deliver this command-and-control structure, in turn, depends on a real commitment by Saudi Arabia, Qatar and Turkey to funnel all military assistance to the rebels through him, not the jihadists.

Here, U.S. diplomatic pressure will be crucial. To empower Idriss, the United States may expand its training and nonlethal assistance to include supplying weapons — even as its real hopes remain with a negotiated peace deal backed by Russia.

For Russia, the Syrian endgame offers a test of President Vladimir Putin’s sincerity and his clout. He regally left the details to Lavrov on Tuesday, after keeping Kerry waiting three hours. This lese-majeste may impress Russians, but it won’t get the job done on Syria. If Putin has finally come to understand that Russia would potentially suffer most from the dissolution of the 1916 Sykes-Picot boundaries in the Middle East, then he will have to put his personal political energy behind the deal, rather than making a handoff to Lavrov.

The extremists also get a vote in this process, unfortunately. Hard-liners within Assad’s camp could step up their use of chemical weapons, hoping to set off a regional bonfire. Sunni jihadists could slaughter Alawites, in revenge for past attacks but also to torpedo a peace deal. Hezbollah and Iran could decide that their interests would be so harmed by Assad’s removal that they would rather torch Syria and take their chances. And Israel could continue its recent attacks, drawing Arab reprisals.

There are many ways this peace initiative could fail. But at least it has begun.

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