Two weeks. Two suicide bombings. Both targeting Shiites in a Sunni land. And both claimed by ISIS.

If this were Iraq or Syria, these attacks – sadly – wouldn't be surprising. But it's not. It's Saudi Arabia, home to Islam's most precious sites and the region's most powerful Sunni rulers -- a relatively vast territory, kept remarkably stable by the ruthless application of authoritarian rule while its neighbors teeter under the destabilizing weight of popular revolution and terrorist intervention.

And that's just the way the U.S. government likes its friend, Saudi Arabia. Because Washington needs stability there more than it needs to feel good about how the House of Saud achieves it.

But today, in Dammam, a city on the Saudi eastern coast, a man dressed as a woman blew himself up outside a Shiite mosque and killed three others. (The attack would have been far more devastating had guards not stopped the bomber from entering the mosque, forcing him back into a parking lot.) ISIS now is bragging that their man reached his target despite heightened security after the group's first attack in the kingdom just eight days ago. That one, on another Shia mosque in a village called al Qadeeh, killed 21.

"They certainly are significant," says Mike Singh, former senior director for Middle East affairs at the National Security Council during the George W. Bush administration. "These attacks seem designed to exacerbate sectarian divisions, precisely as ISIS has sought to do elsewhere."

Singh's right; ISIS wants to encourage Sunni-Shia hostility throughout the Muslim world (perhaps as much as it wants to encourage violence between Muslims and non-Muslims worldwide) because it fits its caliphatic goals.

But for the United States, there's more significance to read into this emerging ISIS assault on Saudi Arabia. And it's the type of significance that should be at least discouraging if not downright worrisome to Washington's Middle East policymakers.

What these attacks say is that Riyadh doesn't have the comforting control over its land that Americans like to believe it does. And if the royal family doesn't have its territory as buttoned down as Washington assumed, what other weaknesses has it been masking? What other vulnerabilities are now on view?

Americans don't like to talk about trouble in Saudi Arabia. That's a little bit because it annoys Riyadh, and really, to what end? The Saudis do enough of America's dirty work in the region to demand some elbow room, corralling the Gulf Arab kingdoms, only occasionally criticizing U.S. military actions, secretly communicating with Israel about shared interests when Washington is napping.

It would be dumb to say it's not also a little bit about oil, but really, that's just the context, not the catalyst for cooperation anymore.

The real reason is that it casts into doubt all of the mythology America has created around its favorite autocratic kingdom. The royal family operates a government that is truly authoritarian. It abuses the rights of its citizens. It discriminates against women. It does frightfully little to protect its minority communities. Beheadings. Disappearances. But, damn, it sure is quiet over there. And man, that's gotta mean those guys are as tough as we need them to be, that they are the mighty Sunni power that's going to help us do one thing in particular -- keep Iran in check.

Indeed, one fantastical American belief about the ceaseless power of Riyadh feeds into another fantastical American belief that Tehran wouldn't dare renege on a deal with the United States. This current argument coming from some quarters that Tehran can be expected to live up to a nuclear agreement is in some part predicated on this belief that Saudi poses enough of a real and present threat to Iran that the Ayatollah won't do anything too destabilizing. (Help Hezbollah, sure. Bomb Jordan, probably not. Use a nuclear energy agreement to build a bomb, nah.)

In fact, these two countries are so inextricably tied that conspiracy peddlers see Iranian meddling in the ISIS suicide bombings in Dammam and al Qadeeh. That's intriguing, but even one of Iran's most ardent detractors in Washington calls it "batty."

Nonetheless, this new Saudi phase of the ISIS battle plan threatens to shatter the illusion of the Saudi behemoth. And that's terrifying. Because without a real and present Saudi threat, the last remaining check on Tehran becomes Israeli spies and American bombs.

If for no other reason, Washington needs to pay attention to what's happening just now in Saudi Arabia.