The next escalation in the Mideast was always a matter of if, and when, US soldiers were killed.

Although the Pentagon lost a couple of Navy SEALs earlier this month in an operation to seize a shipment of Iranian resupplies bound for Yemen, they weren’t killed by enemy fire. (They drowned.)

On Sunday, though, three US service members died in a drone strike on a base in Jordan near the border with Syria. Iranian proxies operating out of Syria and Iraq were responsible, the US said, adding that “many” US personnel were wounded in the attack.

This is bad news. For the dead and wounded, to be sure, but also for Iran. Whatever restraint the Biden administration showed in recent weeks both with regard to the Houthis in Yemen and Iran’s proxies in Iraq, was predicated (implicitly and almost explicitly) on the fact that no US service members had died. Now they have.

Sunday’s incident came a little over a week after Kataib Hezbollah (and friends) launched their most ambitious attack of the current dust-up, targeting the Al Asad Air Base in Iraq with a dozen rockets and half a dozen short-range ballistic missiles. American soldiers and support personnel were evaluated for brain damage. Several days later, Lloyd Austin said the US military conducted “necessary and proportionate strikes” on a trio of facilities in a retaliatory gesture. Note the careful wording: “Necessary and proportionate.”

As of Sunday, the definition of “proportionate” might’ve changed. Biden called the drone attack “despicable” and reiterated America’s “sacred obligation” to the families of fallen soldiers. “We will strive to be worthy of their honor and valor,” he said. “And have no doubt — we will hold all those responsible to account at a time and in a manner of our choosing.”

It’s hard to say exactly which proxy was responsible, but I’m not sure the specifics are especially helpful to casual readers. Simply put: The US is dealing with a handful of breakaways from the PMF which reconstituted themselves under the banner of the “Islamic Resistance in Iraq” some years ago. That umbrella includes Kataib Hezbollah, and Al-Nujaba, which is active in Syria. They’re all supported and backed by Hezbollah (the Hezbollah) and, ultimately, the Quds. On January 4, a US drone killed a senior Al-Nujaba commander who also served as a deputy operations chief for the PMF in Baghdad.

The three Americans killed Sunday represent the first known deaths of the current conflict attributable to Iran’s proxies in Iraq and Syria. It’s a serious escalation. And while I imagine the response, whenever it comes, will still be characterized as “proportionate” by the Pentagon, it’s fair to assume that participating US warplanes will be shooting with the intent to kill somebody, and likely several somebodies.

This comes as the Houthis continue to fire on ships in the Red Sea (including, somewhat amusingly, a tanker carrying Russian fuel) prompting ongoing US airstrikes against targets in Yemen. Frankly, I’m surprised Iran has dialed the temperature as high as it currently is. The regime’s risking a more comprehensive US air campaign against Tehran’s regional assets.

Plainly, Kataib Hezbollah (or a clone) killing three American service members with an unmanned drone won’t prompt US strikes against Iran itself. But when considered with i) the IRGC’s recent missile volley which saw rockets landing in Erbil (among other places) and ii) the Guards’ sometimes shoddy aim, Sunday’s incident does raise the risk of direct confrontation given that any prospective “accident” involving the IRGC itself would be contextualized by this deadly precedent.