First answer me this:
Russia has watched for half a Century as American DemoRat Jimmah Carter created, with American Taxpayer financing the pinnacle Terrorist cesspit that is Iran, from the tolerant Persian People, and the Anglophile Shah.
Then American DemoRats continue to bribe, payoff and coddle that same Iran ending with Obama's pallet loads of cash and an agreement to allow nuclear weapons and missiles to deliver them, without risking an American life to stop their Terrorism, while possessing the Military ability we have now pretty much that entire half Century.
Now we offer the teriffic deal that we won't kill several thousand Russians where they legally reside with that same Military ability, if they fix our dirty work by attacking another of their legal allies of half a Century?
Iran is a threat to U.S. regardless of who started it. The current calculus is based on the current state of affairs, not on the past. Russia has done a lot of bad in the past as well. Is their invasion of the eastern bloc countries 70 years ago reason to stay belligerent with them? No. Neither does what may or may not have happened with regards to Jimmy Carter justify Russia’s current support for Iran. The message to Russia is simple. We don’t really care about Syria, but if you continue to support Iran, then we’ll make it an issue.