The US may not have played a direct role in the multilateral imposition of “de-escalation” or “de-confliction zones” in Syria, but we served notice today that the other parties had better play by their own rules. After pro-Assad forces moved into one such zone and threatened positions held by the anti-ISIS coalition we support, US warplanes conducted airstrikes on the column:

U.S. officials say an American airstrike has hit pro-Syrian government forces in southern Syria as they were setting up fighting positions in a protected area.

The officials say the strike near Tanf hit a tank and a bulldozer and forces there, but it was not clear if they were Syrian army troops or other pro-government allies.

One official says the pro-regime forces had entered a so-called “de-confliction” zone without authorization and were perceived as a threat to U.S.-allied troops there. The officials say the strike was a defensive move to protect the U.S. allies. It wasn’t clear if U.S. forces were there.

Shepard Smith gives a pretty good explanation, along with a rundown of the context:

BREAKING NEWS: U.S. official says U.S. airstrike hits pro-Assad forces in Syria. pic.twitter.com/TSnl4pNhQQ

— Fox News (@FoxNews) May 18, 2017

CNN reports that most of the column had penetrated the no-go zone, and appeared to be looking for a bigger fight later. They claim that the targeted forces were militia rather than regular Syrian military:

Two US aircraft were dispatched as a “show of force” to get the Syrian vehicles to turn around. According to a US defense official, the vehicles did not stop and the show of force was escalated to include an airstrike. The official said at this time it is not clear if the strike was intended as a warning or intended to hit regime forces, who were militia tied to the regime as opposed to regular army troops.

A second US defense official said that an additional strike did hit the convoy after the vehicles continued toward the base.

A convoy of 20 pro-regime vehicles were headed toward An Tanf Wednesday night, searching vehicles and buildings along the way, according to a US official. Thirteen of the vehicles breached the “de-confliction zone” around the base that is the area that the coalition has communicated to the Russians to stay away from. The official said five of the vehicles came within 29 kilometers of the base at about midnight EST Thursday when the aircraft were dispatched.

According to Buzzfeed, the sorties protected American special forces based near Tanf:

As BuzzFeed News reported last week, US Special Forces are deployed at the base with a unit of Syrian rebels. The Special Forces are training the rebels to fight ISIS — and also accompanying them on combat missions against the militants.

But the Syrian regime and its allies — including Iranian forces, Shiite militia, and Hezbollah — are also based some 40 miles outside Tanf, a rebel commander based there told BuzzFeed News last week, allowing them easy access to Tanf via a highway that cuts through the desert.

Unlike the US strike against the al-Shayrat air base in March, which the Trump administration ordered in retaliation for a regime chemical weapons attack, Thursday’s strike seemed designed to protect the US forces based at Tanf.

The US didn’t exactly mince words about the brushback pitch. Our own correspondent Katie Pavlich reports that the Trump administration has loosened the rules of engagement in order for US forces and their allies to work more effectively, and to boost their own security too:

Jennifer Griffin: Rules of engagement have been loosened, authority for strikes being sent back to the field. This is good news.

— Katie Pavlich (@KatiePavlich) May 18, 2017

Trump often criticized the restrictive rules of engagement imposed on American troops in combat areas by the Obama administration during the campaign. “We’re sending leaflets down [saying] ‘in an hour we may be bombing your truck,'” Trump told Fox News thirteen months ago.  “‘Please remove yourself from’ — we are — we don’t know what we’re doing.” The transition team worked specifically on loosening the RoEs almost from the day after the election, and Syria was one of the theaters of special focus for those efforts. The decision to drop the GBU-43, or the MOAB, on ISIS forces burrowed into a mountainside in Afghanistan was the first public manifestation of these new, looser RoEs, and this is the second. It’s clearly meant to send a message.

Now that the message has been sent, however, now what? Syria and Russia didn’t have an immediate response, but both of them have to be concerned over a sudden shift to aggressiveness on the part of the Trump administration. At the very least, they’ll understand that American forces are willing to open fire on any threats to coalition positions, but it might start a tit-for-tat exchange that could escalate rapidly into something else entirely. All sides may need to return to the table quickly to reinforce the separation in forces before that can happen.