AS THIS year began, Syrian ruler Bashar al-Assad was pushing for final victory in the country’s nine-year-old civil war. Backed by Russian warplanes, his forces launched an offensive against Idlib province that seemed likely to crush the last major rebel stronghold. But an unexpectedly robust intervention by Turkey stopped the onslaught, and since then the Assad regime has suffered a series of reverses, including an economic collapse, the resumption of popular protests and, on Wednesday, the initiation of potentially powerful U.S. sanctions.

While the demise of the Middle East’s most brutal dictatorship does not appear imminent, its prospects of stabilizing the country under its rule or reconstructing the economy have suffered a severe setback — as have the strategic ambitions of Russia and Iran. For that, some credit is due to Congress, which mandated the new economic sanctions in last year’s defense bill, and to the Syrian military police defector who inspired them.

“Caesar,” as the defector is pseudonymously known, smuggled 55,000 photographs of people who were tortured and murdered in Syrian prisons and hospitals. His courageous advocacy, including testimony to Congress, eventually led to bipartisan legislation mandating an escalation of U.S. pressure in Syria, including the sanctioning of any foreign actors who provide support to the Assad regime, its air force, its oil industry or reconstruction projects.

The mere prospect of those measures already helped crash the Syrian currency, which has lost two-thirds of its value since the beginning of the year. Desperate to obtain cash, the regime tried to tap the country’s biggest tycoon, an Assad cousin, who instead of complying pushed back, producing a visible rift in the ruling elite. Meanwhile, the squeeze on average Syrians has led to renewed unrest in towns such as Daraa, where the uprising against Mr. Assad began in 2011.

The initial round of Caesar Act sanctions announced by Secretary of State Mike Pompeo was relatively modest, targeting the Assad clique and Iranian militia leaders; most of those named have already been sanctioned. But Mr. Pompeo said that “we anticipate many more sanctions and we will not stop” until the Assad regime agrees to a political solution to the conflict as spelled out by U.N. resolutions. If the Trump administration scrupulously follows the law, that should lead to sanctions against Russian officials and companies backing the Syrian war effort, as well as any foreign companies that buy Syrian oil or help with construction projects.

At best, the effect could be to force Russia and Iran to abandon the Assad regime rather than remain mired in an unwinnable conflict with mounting costs. Those in Congress who backed the legislation, including the chairmen of the House and Senate foreign relations committees, now should ensure that the administration fully follows through on Mr. Pompeo’s promise.

Read more: