All wars end. Syria's violent conflict will stop when combatants have realized an end-state that preserves their core interests. The United States can advance a settlement by presenting a proposal to the country's warring factions, which devolves power from the central government to Syria's regions. Cooperation through the International Syria Support Group (ISSG) is preferable, but the U.S. must be prepared to act on its own if ISSG members, especially Russia, are obstructing progress.
What will Syria look like after the war? The 1980 Taif Accord, which ended Lebanon's civil war, is a model for power-sharing.
A settlement would preserve Syria's sovereignty, while decentralizing authority to an Alawite canton around Damascus and stretching northwest to Latakia. Sunni Arabs would have control in the south and east. Kurds would have domain in the north over Kobani, Afrin and Jazeera provinces. Devolving governance, economic affairs, and local control of natural resources would go a long way to remedy the root causes of conflict.
A settlement must have milestones and a timetable for political transition. Parties may be able to accept Bashar al-Assad as president in the near term. They will, however, demand a date for elections to replace him.
Stabilization will require security guarantees. Security will also allow an orderly and dignified process for displaced persons to go home.
Billions will be needed for humanitarian assistance and reconstruction. The ISSG can evolve into a Peace Implementation Council.
Syrians will demand transitional justice, given the level of human rights abuses and pervasive crimes against humanity. Assad and his Alawite circle will want amnesty arrangements. Others will insist on accountability. Balancing competing demands can be achieved through security sector reform.
The Islamic State and the Nusra Front are the primary spoilers. ISIL is not interested in state-building unless it involves a caliphate. The U.S. will have to intensify efforts targeting jihadists.
International stakeholders are also potential spoilers. The U.S. should be steely eyed, assessing Russia's role. In Syria, Russia is a strategic adversary. Russia fuels conflict because its importance is greater when there is war. Russia's recent bombing of a UN aid convoy exposed its duplicity. Russia must be sidelined if it foments conflict or undermines negotiations.
The U.S. should try to wedge Iran from Russia, using Iran's influence in service of a settlement. However, Iran's support for Hezbollah makes it an unlikely peace partner.
Turkey's invasion and occupation of Syria complicates the path to peace. Turkey will be reluctant to abandon its self-imposed buffer zone.
A durable settlement must require all foreign forces to withdraw, giving way to a peacekeeping operation sanctioned by the UN Security Council. Countries who have supported proxies in the war lack integrity as peacekeepers.
The "ripeness theory" establishes that peace can be achieved when combatants grow weary of fighting and can visualize an end-state that upholds their core interests. According to the ripeness theory, parties are ready to make peace after exhausting unilateral means to achieve their goals. A stalemate on the battlefield, such as the stalemate that exists in Syria today, creates propitious conditions for conflict resolution.
The Dayton Peace Agreement (DPA) was initiated at a moment of ripeness in Bosnia's war when the parties were exhausted by fighting. While there was a contact group institutionalizing international cooperation, the U.S. was the ultimate arbiter.
Diplomacy backed by a credible threat of force was essential to ending Bosnia's war. Since President Barack Obama has made it clear that the U.S. will not get sucked into Syria, a more robust role will have to wait for the next administration.
Ending Syria's national nightmare is possible if the President-elect is clear minded about the end-state and can convince combatants that a settlement is in their interest. Projecting power and influence are integral to U.S. leadership, making America a more effective force for good in the world.
Mr. Phillips is Director of the Program on Peace-building and Rights at Columbia University's Institute for the Study of Human Rights. He served as a senior adviser to the U.S. Department of State under Presidents Clinton, Bush, and Obama, including an assignment as Foreign Affairs Expert in the Bureau of Near Eastern Affairs.