The effectiveness of the Trump administration’s sanctions strategy against Iran has been astonishing—doubly so because the “maximum pressure” policy has faced wide opposition, including from close U.S. allies. Most foreign companies have decided to avoid doing business with Iran rather than face potential U.S. secondary sanctions, and this week Britain, France and Germany joined in blaming Tehran for the recent missile attack on Saudi Arabia.

Iran complains bitterly about Mr. Trump’s policy, but by violating its nuclear commitments and attacking its neighbors, it practically invites the U.S. to do more. One option is to put the full weight of international law behind its sanctions campaign—unilaterally.

President Obama’s sanctions rested largely on six binding resolutions adopted by the United Nations Security Council between 2006 and 2010. They were terminated in 2015 by Resolution 2231 implementing the nuclear agreement, known as the Joint Comprehensive Plan of Action.

Under Resolution 2231’s “snapback” provisions, any JCPOA participant can file a complaint with the Security Council about “significant non-performance of commitments” by another participant. That triggers a 30-day process, at the end of which all provisions of previous Security Council resolutions are reimposed unless the council adopts a resolution to the contrary—which would be subject to the American veto.

Thus the U.S., acting alone, can both trigger this provision and ensure that sanctions are reimposed. In December the U.S. will hold the Security Council presidency, putting it in a position to manage this process.

Some might argue that because the U.S. withdrew from the JCPOA, it’s no longer a “participant” and therefore cannot trigger this procedure. In fact, Resolution 2231 defines “JCPOA participant” to include the U.S. without qualification.

Despite their likely qualms about restoring U.N. sanctions, most Western countries can be expected ultimately to comply with clear international law, especially since Iran is progressively pulling itself out of compliance with the JCPOA. Russia and China may not, and it would be regrettable if they began openly defying binding U.N. Security Council mandates. But if the Trump administration were preoccupied with international law and institutions, it wouldn’t have abandoned the JCPOA in the first place.

Reimposing the terminated U.N. Security Council resolutions would offer many benefits from the administration’s perspective. The U.S. policy of “maximum pressure” would be backed up by multilateral, U.N.-imposed sanctions. The scope for international resistance to U.S. policy—under the European Union’s “blocking statute,” for example—would be narrowed. Important JCPOA benefits Iran has been counting on—such as expiration of the U.N. arms embargo in October 2020—would vanish.

Equally important, the U.S. would have enhanced leverage in any new negotiations. The baseline would no longer be the JCPOA, which conceded Iran’s right to operate uranium-enrichment and ballistic-missile programs, but previous U.N. resolutions that prohibited Iran from operating such programs.

Originally published in The Wall Street Journal on September 25, 2019.