That’s fair, but it would also show their true intentions and turn the world against them. As seen with Russia, the world doesn’t like it when you commit atrocities, and they’ll band together to make it more difficult for you. As a result, I don’t see them actually using nukes on Iran. Nukes have always been a better deterrent than a weapon.
I expect that they would keep Iran at bay by doubling down on their rhetoric to get the most out of their nuclear deterrent (just like Russia has). But being the smaller nation (by a lot) and also being generally outnumbered in the region by Islam, I don’t see them invading Iran, the odds would be impossibly against them.