If you don’t know why Iran would be worse, find out. They are still the world’s number one state sponsor of Islamic terrorism, and they are insistent upon starting World War III to induce the coming of the Mahdi.
And there was general unity during the Crusades against the Christian world that transcended any Sunni-Shi’a dispute about a born successor to Muhammad. That was mostly due to the numbers of Sunni to Shi’ite, but that would still apply today—especially since all of Iran’s Supreme Leaders were and are about putting all differences aside to wage jihad against the non-Muslims.
I think the Shia-Sunni division is strong enough to keep them fighting. At least I hope so.
The Shia are pretty heavily outnumbered and out-moneyed though and will, probably, eventually lose or be bought off.