Iran is a country of 75 million people with a high GDP that has been funneled into building a modern military. Not only are they well supplied by the Russians but they have the capabilities to manufacture their own military hardware. They have advanced radar and air defenses, Chinese tanks, Russian built submarines. They have the 8th largest active army in the world with the 6th largest reserve force.
A war against Iran without Russian backing would make the wars in both Afganistan and Iraq look like a bit of mild sparing.