THE LATEST partial truce in Syria got off to a bad start Monday, with the regime of Bashar al-Assad reported to be bombing and shelling the very areas the deal is supposed to cover. Whether the truce will ever get off the ground will likely depend on whether Moscow can restrain its client dictator, who hours before the cease-fire began repeated his vow to recapture all of Syria by force. But Vladi­mir Putin’s regime at least has a motive to succeed: If it does, it will have realized Mr. Putin’s aspiration of imposing his will on the United States.

When Russia launched its direct military intervention in Syria a year ago, President Obama predicted its only result would be a quagmire. Instead, the agreement struck by Secretary of State John F. Kerry on Friday with his Russian counterpart offers Mr. Putin everything he sought. The Assad regime, which was tottering a year ago, will be entrenched and its opposition dealt a powerful blow. The United States will meanwhile grant Mr. Putin’s long-standing demand that it join with Russia in targeting groups deemed to be terrorists. If serious political negotiations on Syria’s future ever take place — an unlikely prospect, at least in the Obama administration’s remaining months — the Assad regime and its Russian and Iranian backers will hold a commanding position.

In exchange for these sweeping concessions, which essentially abandon Mr. Obama’s onetime goal of freeing Syria from Mr. Assad and make the United States a junior partner of Russia in the Middle East’s most important ongoing conflict, Mr. Kerry promises that humanitarian lifelines will be opened into the besieged city of Aleppo and other areas subjected to surrender-or-starve tactics. The Syrian air force will supposedly be banned from dropping “barrel bombs,” chlorine and other munitions on many areas where rebels are based — though there seem to be loopholes in the deal, and its text has not been made public.

If that really happens, and lives are saved, that will be a positive benefit. Perhaps it’s the only one available to a U.S. policy that swears off, as doomed to failure, the same limited military measures that Russia has employed with success. But Mr. Putin and Mr. Assad have agreed to multiple previous truces, in Syria and, in Mr. Putin’s case, Ukraine — and violated all of them. Their reward has been to gain territory and strengthen their strategic positions, while receiving from the United States not sanction but more concessions and proposals for new deals. If the regimes observe their promises in this case, it may be because the time to exploit this U.S. administration — which has retreated from its red lines, allowed Russia to restore itself as a Middle East power and betrayed those Syrians who hoped to rid themselves of a blood-drenched dictator — is finally running out.