IT’S BEEN MORE than a year since the Obama administration began describing the downfall of Syrian dictator Bashar al-Assad as a matter of time. He’s defied White House predictions in part because of his ruthlessness — more than 20,000 Syrians, most of them civilians, have died during the uprising — and in part because of political and military weakness of his opposition.

But the Assad regime also has been bolstered by the imbalance of foreign intervention. While the United States and other Western powers hang back, and Turkey, Saudi Arabia and Qatar offer weak and poorly coordinated aid, Iran has mounted a concerted and escalating campaign to prop up the government. Iran has sent men as well as arms, cash and technical support.

White House aides are frequently described as worrying that direct U.S. aid to the rebels would intensify the war — which is now bloodier than Iraq at the height of its sectarian fighting — or prompt other countries to jump in. Ayatollah Ali Khamenei’s regime has no such scruples. The commander in chief of the Revolutionary Guard, Mohamad Ali Jafari, acknowledged at a news conference last weekend that members of the elite Quds force have been training a 50,000-member civilian militia modeled on Iran’s own Basij force. Those fighters, known to Syrians as shabiha, have been guilty of some of the war’s worst atrocities, including the mass murder of civilians in several villages.

The Wall Street Journal reported that the Revolutionary Guard general leading the training effort, Hossein Hamedani, commanded the assault on Iranians who revolted against the regime in 2009. Hundreds of rank-and-file members of the Guard and Basij are in Syria to bolster government forces, the Journal said. A busload of 48 was captured by the Free Syrian Army last month.

Even as the Obama administration was rejecting appeals from Turkey, France and other allies for more robust action against the Assad regime, Tehran was escalating. In July it resumed direct cargo flights to Damascus across Iraqi airspace. U.S. officials, who believe that the planes carry military equipment, appealed to the Iraqi government to stop them or at least inspect them. But the flights have continued. U.S. officials told the New York Times that Iran has even provided the regime with a cargo plane for transporting fighters and supplies around the country. Meanwhile, the Lebanese militia Hezbollah, an Iranian client, has been sending its own trainers into Syria.

Some administration officials dismiss the Iranian effort as futile support for a lost cause. But Iranian backing for the regime, matched against Western passivity, could keep Mr. Assad in power indefinitely. Even if the government in Damascus collapses, Iranian commanders and the militias they’ve trained will likely stay on to compete in what could be a chaotic struggle for power that could spread from Syria to Lebanon and Iraq. Al-Qaeda and other Sunni extremist groups will be part of that fight; so will the rebel groups backed by the fundamentalist Saudis and Qataris. If it continues its present policy, the United States will go on watching from the sidelines as the future of the Levant is decided.