Nytimes.com | By Peter W. GALBRAITHDEC - TOWNSHEND, Vt. — The civil war in Syria is over. Now it is time to stop the fighting.

Aided by Russia, Iran, Shiite militias and Hezbollah, the government of President Bashar al-Assad is on the verge of taking Aleppo, once Syria’s largest city. Supported by its powerful allies, the Syrian Army will then move to eliminate the remaining pockets of resistance, notably around the northern city of Idlib. While Iran has been Mr. Assad’s most important military ally, the Syrian regime would still want to have Russian airpower to finish its reconquest of the country’s populous west.

The Assad regime has prevailed through tactics of unspeakable brutality — barrel bombs, starvation, the targeting of hospitals and rescue workers and the suspected use of chemical weapons — but it has prevailed. Samantha Power, the United States ambassador to the United Nations, has rightly focused attention on these war crimes, but these denunciations will make no difference to the situation on the ground.

There is an absolutely counterproductive idea favored by Washington’s foreign policy elites of both parties, recycled recently by President Bill Clinton’s secretary of state Madeleine K. Albright and Stephen J. Hadley, national security adviser to President George W. Bush, for providing additional military support to the moderate Syrian opposition. Such aid cannot possibly now change the trajectory of the war, but will certainly get more people killed.

Though the outcome is clear, how the war ends matters greatly. The United States has an interest in a result that allows as many Syrians as possible to go home, that ensures the total defeat of the Islamic State and other extremist groups, and that safeguards the Syrian Kurds, who have been America’s principal ally against the Islamic State.

Achieving these goals will require close collaboration with Russia, whose intervention enabled Mr. Assad to turn the tide of the war. Fortunately, Russia shares many of America’s objectives, even if its Syrian ally does not.

The United States and Russia could start by negotiating terms that would end the fighting between the regime and the moderate opposition. The terms might include an amnesty for the rebels, the right of Syrian refugees to return and equal access to reconstruction assistance. It could even include some promises of basic political freedoms, international monitoring and the removal of Syrian officials (not including Mr. Assad) responsible for the worst crimes.

The Russians have considerable leverage with a Syrian government that wants Russian backing for mopping-up operations. The United States, with less leverage, will have to persuade the non-Islamist opposition that a negotiated surrender is better than total destruction.

European countries have a strong interest in creating conditions to encourage refugees in Jordan, Lebanon and Turkey to return to Syria rather than heading west. America should work to ensure the diplomatic engagement of European allies to bring an end to hostilities, as well as their financial support for reconstruction in Syria.

In eastern Syria, Kurdish forces supported by the United States Air Force and special forces are battling the Islamic State in a largely separate conflict. On a recent trip to the Kurdish areas, I traveled to within 15 miles of Raqqa, the capital of the Islamic State. Kurdish fighters feel confident that they can take the city, but their leaders understand that they’re not in a position to govern a large Arab city. Since there is no viable Arab alternative to the Syrian government, this will mean transferring control of Raqqa to the regime in Damascus.

Finally, the United States must provide long-term guarantees to the Syrian Kurds, who now control a large territory, not all of which is Kurdish. For now, the Syrian Army is in no position to take on the Kurdish forces, but eventually, Mr. Assad will surely try to recreate the centralized Arab state he inherited from his father. He will also want to use Syria’s oil resources — much of which are now under Kurdish control — to finance reconstruction.

One option is to establish an American-protected Kurdish safe area in northeastern Syria similar to the one created in northern Iraq after the first gulf war. That expensive option is complicated by the inability of the United States to use Turkish air bases to enforce it. (Turkey regards the Kurds as its leading enemy in Syria.) The less costly alternative is to co-sponsor a Russian plan for an autonomous Kurdish area within a federal Syria.

However, Russia’s leverage with Mr. Assad will diminish as the opposition crumbles in Syria’s west and Russian airpower becomes less important. At that point, the opportunity to extract concessions will disappear, and the field will belong to Mr. Assad and Iran.

President-elect Donald J. Trump has stated his intention to work with Russia and Mr. Assad to defeat the Islamic State. The sooner America reaches out to Russia, ideally before January’s handover of administration, the better.

Peter W. Galbraith is a former United States ambassador to Croatia.