When the revolt in Syria began in 2011, many policy analysts and former officials argued that the downfall of the Assad regime would be a major setback to Iran. I was one of them, and the claim was not complicated: Syria was Iran’s only Arab ally, provided its only ports on the Mediterranean, was a land bridge to Hezbollah in Lebanon that allowed Iran an easy means of arming Hezbollah, and via Hezbollah gave Iran a border with Israel. The fall of Assad would deny Iran all these assets and all these possibilities.

That last notion—that Iran would have a border with Israel—was of course partly a rhetorical point. In 2011, there were no Hezbollah forces fighting in Syria, nor of course were there any Iranian troops there. And it was hard to imagine that this would ever happen, or that the United States would actually permit it. An Iranian Revolutionary Guard expeditionary force, fighting first in Iraq and then in Syria? Iranian troops nearing Israel’s border in the Golan? And even harder to imagine was all this being done without American resistance—and apparently with American agreement.