One hundred and thirty thousand Syrians have already died. More than 2 million are displaced. Many are at risk of starvation. Polio is breaking out. The best thing the United States can do for Syrians, by far, is to reach a nuclear deal that ends its cold war with Iran.

Iran has good reason to want the carnage in Syria to stop. Propping up Bashar al-Assad is costing Tehran’s sanctions-ravaged regime financially. Iranian Revolutionary Guard forces are dying on Syrian soil. Most importantly, the war has devastated Iran’s reputation. As a Persian, Shia power, Iran’s Islamic regime has traditionally looked for causes—like the Palestinians—that give it credibility in the Sunni-dominated Arab and Muslim world. Tehran’s support for Assad’s murderous repression of Syria’s Sunnis has done the exact opposite. Iran’s approval rating in 20 Muslim-majority countries, according to Zogby Research Services, has plunged from 75 percent in 2006 to 25 percent today.

So what’s preventing Iran from backing a process that eventually leads to Assad’s exit? According to Iranian-American journalist Hooman Majd, author most recently of The Ministry of Guidance Invites You to Not Stay, Iran’s militant Revolutionary Guards see Assad’s Syria as a key base for projecting Iranian power. Others in the Iranian regime are less convinced of Assad’s value, but fear that the forces that oust him will use Syria as a beachhead to try to oust them. The United States cannot entirely allay that fear since acolytes of al-Qaeda now dominate the militias fighting Assad. Still, it would be easier for Iran’s regime to support elections that produced a Sunni-led Syrian government if the U.S. promised to try to prevent that government from expressing virulent hostility to Iran. And the U.S. can’t credibly make that promise if Iran’s leaders think Washington is virulently hostile itself.