There are many more Sunnis than Shiites. The traditional balance of power was between the Sunni rulers of the oil states and the Shiite rabble in the slums in those states. Saudi, Bahrain, Qatar, UAE are all set up like that. Libya was, and Iraq was to some extent. The exception was Iran where the rabble took over and formed their own fundamentalist state although more modern in many ways. When you see a medieval Muslim beating his wife with a stick think Sunni. When you see a modern Muslim with a cell phone blowing himself up, think Shiite. Oddly the Shiites will beat themselves too.
Next the Sunnis invent ISIS which is essentially Taliban on steroids funded by the Sunnis in Qatar and elsewhere. ISIS is fundamentalists with knives slicing up the fundamentalists with cell phones. ISIS will use cars but only to get to new places faster to kill more people. The Muslim brotherhood is Sunni. Obama is Sunni like his masters in Saudi. Obama did not want to make a serious effort to degrade ISIS since he is on their side.
The problem for Obama is that ISIS is a bit like Nazi Germany taking over swaths of territory and making his religion look bad. He is being forced to ally with Iran to degrade them if there is political pressure to do that. If he does and Shiite Iran emerges victorious (along with their ally Russia), then we will have new cold war with a nuclear Shiite Iran. Iran will fund terrorist Shia rabble in places like Turkey and Pakistan against the Sunni majority and rulers there.
Obviously we don't want a strong nuclear Iran as an outcome. But we also don't want a nuclear ISIS centered in Syria and Iraq. There is no good solution but waging a large war on the side of Iran is one of the poorer choices.
Personally, I think that this whole ISIL issue is a scam to get Iran to cross the border, facilitating a strike against the from the Sunnis across the gulf (and us.)
We will do very little until he absolutely have to.