To date, the Y.P.G. has been little more than a pawn of the two superpowers, but the way the situation is evolving suggests an opportunity for the United States and Russia to cooperate with the Kurds to create an alternative future for some Syrians.

Last week, the United States and Russia announced a “cessation of hostilities” between some of the main factions in Syria. The Syrian government also signed on to this deal, which excluded extremist groups like the Islamic State and Al Qaeda affiliates, but the agreement’s success is far from assured. Mutual interests with the Syrian Kurds could become the glue that holds it together. But what would such a collaboration look like?

The United States should reinforce its military support for the Syrian Democratic Forces, but on the condition that the coalition continues to expand its multiethnic credentials by incorporating moderate Sunni groups. This would enable the United States to deflect Ankara’s claims that the Americans are giving aid to P.K.K. terrorism.

The democratic and pluralist goals of the S.D.F. make it a potentially useful Western ally in any future federalized Syria. At the same time, the S.D.F.’s relatively neutral stance toward the Assad regime helps it gain Russian support, both political and military.

Russia could use its influence with Damascus to ensure that Syrian government forces do not attack the S.D.F. The federalist model proposed by the S.D.F. also offers Russia options for a post-Assad transition of power. Another advantage of a federalized Syria is that neither Russia nor America would have to face the political challenges of completely partitioning the country — a possibility Secretary of State John Kerry warned of on Tuesday. A federalist solution would help to head off the risk of an independent Kurdish state’s emerging in northern Syria.

Russian-American cooperation over the S.D.F. faces one serious stumbling block: Turkey. As well as increasing the prominence of Sunni militias within the group, the United States will also need to use diplomatic channels and its commanding position in NATO to make it clear to Mr. Erdogan that Turkey will not enjoy American support if it engages in further military intervention in Syria or clashes with Russia.

The Syrian conflict is beyond messy. But the S.D.F. gives America and Russia an opportunity to act on convergent aims, and suggests at least the tentative outlines of a lasting peace.