Iran and Russia move closer but their alliance has limits

Russia's President Vladimir Putin (L), who arrived to attend the Gas Exporting Countries Forum (GECF), meets with Iran's Supreme Leader Ayatollah Ali Khamenei in Tehran, Iran, November 23, 2015

When Iran took delivery of the first parts of an advanced Russian air defense system this month, it paraded the anti-aircraft missile launchers sent by Moscow to mark Army Day.

Tehran had cause to celebrate: the Kremlin's decision a year ago to press ahead with the stalled sale of the S-300 system was the first clear evidence of a growing partnership between Russia and Iran that has since turned the tide in Syria's civil war and is testing U.S. influence in the Middle East.