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.
The use of Force.