Since the outbreak of the Syrian civil war in 2011 , Iran and Russia have become the Syrian government 's principal allies in the conflict , openly providing armed support . Meanwhile , Russia 's own relations with the West plummeted due the Ukraine crisis , the 2018 Skripal poisoning incident in Great Britain , and alleged Russian interference with Western politics , prompting the U.S. and Europe to retaliate with sanctions against Russia . As a result , Russia has shown a degree willingness to ally with Iran militarily . Following the JCPOA agreement , President Vladimir Putin lifted the ban in 2015 and the delivery of the S-300 missile defense system to Iran was agreed to . The delivery was completed in November 2016 and was to be followed by a $ 10 billion deal that included helicopters , planes and artillery systems
2018 Skripal poisoning incident
Poisoning of Sergei and Yulia Skripal
Iran–Russia relations