On Thursday, the United States military bombed Iranian-controlled forces in southern Syria. It also shot down one of their drones.

With three tweets, a U.S. spokesman explained what happened.

"U.S. conducted strikes against two technical vehicles that we were assessed to be posing a threat to Coalition forces at At-Tanf garrison."

"Today was the third set of kinetic strikes the U.S. conducted in response to threats posed to Coalition forces operating out of At-Tanf."

"The Coalition does not seek to fight Syrian regime or pro-regime forces. However, we remain ready to defend ourselves against any threat."

As the spokesman noted, it's the third time this has happened. It's a big deal.

At-Tanf is a base for the counter-Islamic State coalition. Located near Syria's intersecting border with Jordan and Iraq, At-Tanf sits on a major highway linking southern Syria and Iraq. That gives it critical strategic utility for the British and U.S. special forces stationed at the base. Those forces are training moderate Syrian rebel forces fighting the Islamic State and Syrian President Bashar Assad's regime. The special forces probably also stage from At-Tanf in their own missions against the Islamic State.

But considering the garrison's anti-Islamic State mission, some might ask why Iran is endangering these forces?

After all, Iran also opposes the Islamic State. At least, that's what Iran claims.

The reality is murkier.

Put simply, even after Wednesday's attacks in Tehran, the Islamic State remains a peripheral Iranian concern.

Iran's priority lies in defeating rebel groups fighting Assad. And while Iran and its allies are making progress towards that ambition, the U.S. military's At-Tanf garrison is an obstacle.

That's because At-Tanf enables the U.S. to influence future developments in eastern Syria. That's because it allows for future U.S. support to the Sunni tribes who live on either side of the central Syria-Iraq border. Those tribes oppose Iran's efforts to establish regional Shia hegemony. Iran wants to prevent their empowerment.

A good example of how Iran carries out this agenda is found in its actions against the tribes of Iraq's Anbar province. Acting through its interlocutors and agents in Baghdad, the Iraqi capital, Iran has systematically degraded Anbari political empowerment and independence.

Of course, that's precisely why the U.S. must not yield when it comes to the At-Tanf showdown.

Ultimately, At-Tanf is just a metaphor for the broader regional struggle between multi-sectarian cooperation and political sectarianism. The former fosters peace and stability, the latter fuels hatred and terrorism. For proof of that dichotomy, one need only look at Lebanon, Iraq, Qatar, or Iraqi-Kurdistan. Those situations speak to a hard truth.

Namely, that while Iran and the Islamic State might be enemies in action, they are allies in effect. Every time Iranian agents kill or repress Sunnis, Sunni jihadist groups like the Islamic State benefit. Iran's malfeasance allows the Islamic State to sell the message: "Join us or live in slavery." In the same way, every time the Islamic State kills or represses Shia innocents, Iran's power grows. The Islamic State and Iran need each other to support their pathetic ideologies.

Challenging this sectarianism is crucial. Absent that, alongside declining oil prices, Middle Eastern politics will continue to be defined by violent extremism. And as we know from Orlando, Paris, and London, it's an extremism unconstrained by international borders.