“I could see as part of this deal [between Tehran and Moscow] that they would agree to transfer advanced missiles to Iran,” said Mark Dubowitz, the executive director of the hawkish Foundation for the Defense of Democracies and an expert in the Iran sanctions. “If [Russian president Vladimir] Putin became angry enough over the West’s financial punishment of Russia, he could put in play the S-300 deal.”

The S-300 has long been Moscow’s top-of-the-line air defense system. The current model is comparable to U.S. Patriot missile batteries. The S-300 deploys sophisticated radars, launch vehicles and missiles to shoot aircraft and even ballistic missiles out of the sky. Russia has also threatened to sell the system to Syria, whose hapless air force was hacked by Israel in 2007, rendering its anti-aircraft defenses useless when Israel bombed the al-Kibar nuclear facility.

In the second term of the George W. Bush, Russia came close to selling and training Iran’s military on how to use the sophisticated S-300 system. But then in 2010, the Russians pulled back from the sale during negotiations over U.N. Security Council resolution 1929, the resolution the Obama administration used to persuade banks and finance ministries all over the world to isolate most of Iran’s economy.