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.