I’m not certain they’ll need to invade Iran. They’ve already got a mutual defense pact with Iran and it’s Russian expertise and materials driving the Iranian nuke program. One thing’s for certain, with a Russian presence in Iran and Syria, Iran/Syria/Hezbollah will being particularly emboldened. This really affects Israel and her Iranian planning.
The alternative is a hostile nuclear armed fanatical islamist jihadist state at their underbelly.