The militias are fighting alongside Syrian government forces to keep Assad in power. But officials think Iran’s long-term goal is to have reliable operatives in Syria in case the country fractures into ethnic and sectarian enclaves.

A senior Obama administration official cited Iranian claims that Tehran was backing as many as 50,000 militiamen in Syria. “It’s a big operation,” the official said. “The immediate intention seems to be to support the Syrian regime. But it’s important for Iran to have a force in Syria that is reliable and can be counted on.”

Iran’s strategy, a senior Arab official agreed, has two tracks. “One is to support Assad to the hilt, the other is to set the stage for major mischief if he collapses.”