But Russia, like much of the world, has never been too keen on cutting off trade and finance with Iran. Iran's oil wealth is too sweet to resist, even as it works to build a nuclear bomb that could disrupt an already-precarious Middle East.

But America's inability to line up partners is just one weakness in its strategy for isolating Iran. Sanctions are only effective if they are enforced. And U.S. officials in charge of choking off illegal trade with Iran have proven feeble. For years, Iran has used shell companies and complicit banks to spirit billions of dollars through New York alone, evading sanctions.

Take London's venerable Lloyds TSB bank. It recently admitted to laundering billions of Iranian dollars through Manhattan. Branches abroad accepted deposits of Iranian rials from Iranian state banks — including some with alleged ties to Iran's nuclear program. A Lloyds unit would then remove information identifying its rogue client from electronic records and transfer the money to New York.

The service was invaluable. The dollar is still the currency used in most international trade, including the black market. Lloyds only fessed up under pressure from Manhattan District Attorney Robert Morgenthau. That's right: Not federal Treasury officials or the FBI, but Manhattan's top prosecutor.

To avoid prosecution, Lloyds opened its books to federal and New York authorities. They are trying to determine whether any of the laundered money can be tied to Iran's weapons program. But it doesn't stop at Lloyds. Morgenthau told me his office is probing several other New York institutions, and said to expect charges against an even bigger bank this fall.

It might be surprising that a case with such international importance fell onto the lap of a district attorney. Indeed, Morgenthau has made pursuing white-collar crimes a signature of his long tenure. But some say he's also filled a gaping hole left by federal officials ill-prepared and insufficiently trained to pursue these cases.

It's become a cliche, but "follow the money" is also one of the central tenets of law enforcement (and investigative reporting). Doing so requires good sources, access to documents and personnel trained to notice irregularities. But reliable intelligence on Iran has been notoriously weak since diplomatic relations were severed after the 1979 hostage crisis. The White House picture of Iran is a pointillist collection of foreign news reports and informants abroad. President George W. Bush made clear his frustration about the lack of reliable information on Iran's nuclear program. Near the end of his presidency, he somewhat desperately opened an "Iranian listening post" in nearby Dubai. From an office tower, a team takes the pulse of the United Arab Emirates' half-million Iranians.

Meanwhile, back at home, an aggressive new Treasury Department effort has led to a growing blacklist of sanction-busters tied to Iran. Some of the names on the list have come directly from Morgenthau's investigators. Like the name of Chinese national Li Fang Wei and his company Limmt Metallurgy and Minerals Co. The district attorney recently named Li in another case involving illicit Iranian trade with what appears to be more direct ties to Tehran's weapons program. Li, identified as an arms trader, was charged with using more than a dozen front companies to purchase materials for Iranian missiles.

"They're making every effort to do this through deceit," Morgenthau said in April. "We want to have transparency in these dealings so that everybody understands what the Iranians are up to."

Thanks to Mr. Morgenthau, we now know what the Iranians are up to. But Washington still doesn't seem to know how to stop them.