The U.S. must not allow Iran to seize Kurdish border crossings between Iraq and Syria.

Because Iran is attempting to do just that.

After a few days of fighting, Iraqi forces and Iranian-controlled Popular Mobilization Force (PMF) militias have secured the Iraqi-Syria border crossing at Rabia. They now seem set to push north towards the remaining Kurdish border posts up to the Turkish tri-border area near Faysh Khabur. It's only a 25 mile stretch, but as this map shows, the offensive would split Kurdish territory between Iraq and Syria.

That would afford Iran a huge strategic victory.

That victory wouldn't simply be in preventing the Kurds from moving personnel and supplies, but in subjecting the U.S. to those same restrictions. Iran will thus be able to extract political concessions in return for allowing border crossings; whether humanitarian in nature or otherwise.

Moreover, excluding the Anbari crossing points further south (which Iran is also fighting to secure), Iranian success in the north would construct a contiguous supply line between Tehran and Beirut. And that would mean Iran’s unimpeded ability to move personnel and materiel such as rockets throughout the Levant. While Iraqi federal forces might retain presumptive control over border crossings, Iran's de facto control via the PMF's influence would entrench its influence in Iraqi politics.

The boldness here is not coincidental.

On the contrary, this is just the latest stage of a Russian, Iranian and Turkish effort to carve out a new political reality in Iraq and Syria. It would fuel Sunni sectarian hatreds that empower the Islamic State, it would mean the subjugation of the Kurds and an end to multisectarian political efforts in Baghdad, and via Iranian supply corridors it would pose a grave threat to Israel.

The U.S. cannot accept those outcomes.

First, President Trump must endeavor to persuade Iraq's prime minister to suspend any federal force involvement in offensives towards Faysh Khabur. The key here should be to separate Iraqi government forces from the PMF and to get Baghdad talking with the Kurdish regional government. So as to reduce Abadi's political vulnerability to faux-nationalist, pro-Iranian hardliners in his Dawa party, the U.S. should also pressure Masoud Barzani to step down as the president of Iraqi Kurdistan (Barzani's credibility with Kurds of all political stripes is now plummeting anyway).

Still, the U.S. should be prepared to use military force to ensure the remaining Kurdish-held border crossings do not fall. The PMF recognize their vulnerability to U.S. air power and likely can be deterred from continuing north. If not, the U.S. must be ready to bomb them back into their box. Simultaneously, Trump should use his "great friendship" with President Recep Tayyip Erdogan to tell him that the U.S. won't tolerate an invasion from the north to finish the job for Iran.

Earlier this month, Trump pledged that the U.S. would no longer turn a blind eye to Iranian malevolence in the Middle East. We're about to see if he meant it.