As conflict spiraled and casualties increased, so would international pressure for another costly, protracted and thankless American-led ground intervention to enforce peace, which domestic opinion in the United States would not support. While Russia’s real appetite for a political solution in the Syria conflict is unclear, it is wiser to test unknown political limits than unknown military ones.

Some of those advocating more intervention in Syria believe that as the so-called indispensable power, the United States has an ethical responsibility to reduce the suffering caused by Syrian and Russian bombing of civilians. Another camp of interventionists criticizes what it sees as President Obama’s weakness, heartlessness and strategic myopia, and wants the United States to stand up to Russia and assert its intention to remain a major geopolitical player in the region.

The liberal interventionists seem to have forgotten that it is no longer the 1990s. Disastrous forays in Iraq and Libya have undermined any American willingness to put values before interests. Meanwhile, the second group of interventionists seem to have forgotten that Syria has been Moscow’s client since early in the Cold War — a situation Washington was willing to live with when the geostrategic stakes were much higher.

The United States does, in fact, have a clear Syria policy: Roll back the Islamic State by way of the air campaign and American-supported Syrian rebel forces, coordinating with Russia to the extent possible; provide extensive humanitarian support; and continue to press for a sustainable cease-fire and a negotiated political transition involving Mr. Assad’s eventual departure. It may be frustrating, but against the alternatives, it is the only sensible course of action.

Certainly, the Syrian government and Russia have manipulated the cease-fires, using them as cover for continuing offensives. Nonetheless, fragile though they have been, these deals ratchet down the overall level of violence and save lives.

The deal struck by Mr. Kerry and his Russian counterpart, Sergey V. Lavrov, for the United States and Russia to coordinate counter-jihadist operations and restrain opposition and government military activity was intended to produce a durable cease-fire, promote more effective humanitarian operations and re-energize political talks. Although it has fallen apart, the next step, unsatisfying as it may be, is to try again.