Just three days after Turkish warplanes killed at least 20 US-backed Kurdish fighters along the Turkey-Syria border as well as several Kurdish peshmerga troops on Mount Sinjar in northwestern Iraq, footage posted by Syrian activists showed the US has deployed troops and APCs in the contested region, in a move that could potentially drag the US in a conflict where it already finds itself mediating between two so-called US ally forces in the proxy war against Syria.

The Turkish airstrikes also wounded 18 members of the U.S.-backed People's Protection Units, or Y.P.G., were criticized by both the U.S. and Russia. The YPG is a close U.S. ally in the theatrical fight against the Islamic State (whose real purpose is destabilizing the Assad regime); it is seen by Ankara as a terrorist group because of its ties to Turkey's Kurdish rebels. The problem is that Turkey is also an ally of the US, although over the past two years relations between Turkey and all western NATO allies have deteriorated substantially for numerous familiar, and extensively discussed in the past, reasons.

On one hand, further clashes between Turkish and Kurdish forces in Syria could potentially undermine the U.S.-led war on the Islamic State group. On the other, it risks taking an already unstable situation in Syria and escalating it substantially, should Turkey again find itself invading Turkey and/or Iraq.

Which is why the US appears to have deployed troops along the border: to serve as a deterrent to further Turkish attacks.

A senior Kurdish official, Ilham Ahmad told The Associated Press that American forces began carrying out patrols along the border Thursday along with reconnaissance flights in the area. She said the deployment was in principle temporary, but may become more permanent. Another Kurdish activist said the deployment is ongoing, adding that it stretches from the Iraqi border to areas past Darbasiyah in the largely Kurdish part of eastern Syria.

"The U.S. role has now become more like a buffer force between us and the Turks on all front lines," he said. He said U.S. forces will also deploy as a separation force in areas where the Turkish-backed Syrian fighting forces and the Kurdish forces meet.

As noted above, the US intervention is meant to send a "a message of reassurance for the Kurds and almost a warning message" to the Turks, he said.

Navy Capt. Jeff Davis, a Pentagon spokesman, did not dispute that U.S. troops are operating with elements of the Syrian Democratic Forces (SDF) along the Turkish border, but he would not get into specifics. The SDF is a Kurdish-dominated alliance fighting IS that includes Arab fighters.

"We have U.S. forces that are there throughout the entirety of northern Syria that operate with our Syrian Democratic Force partners," Davis said. "The border is among the areas where they operate." He said the U.S. wants the SDF to focus on liberating the IS-held town of Tabqa and the extremist group's de facto capital, Raqqa, "and not be drawn into conflicts elsewhere."

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Confirming that the proxy war in Syria is becoming ever less so, the U.S. has recently shifted from working quietly behind the scenes in Syria's conflict toward overt displays of U.S. force in an attempt to shape the fight. Last month, about 200 Marines rolled into northern Syria backed with howitzers, significantly widening America's footprint in a highly toxic battlefield. The Marines' deployment came days after another intervention, when dozens of army troops drove outside the town of Manbij, riding Stryker armored vehicles, following an earlier conflagration of fighting between Syrian Kurdish troops and Turkish troops. The U.S. deployment in Manbij intentionally put Americans in the middle of that rivalry, hoping to cool it down.

The SDF retook Manbij from IS control, and Turkey said it won't allow the town to be under Kurdish control, threatening to move on it. The American presence appears intended to reassure Ankara the Kurds don't hold the town.

But the new deployment puts U.S. troops directly along the border with Turkey, another flashpoint, and immerses Washington into that increasingly hot fight. Should Erdogan happen to launch a strike against a zone containing US troops, he can simply say he was aiming elsewhere, although the retaliation by his NATO ally would be prompt.

It remains unclear if the US is now actively seeking to engage Turkey on the combat field, and is looking for a politically correct, and media friendly pretext to do so. It is also unclear what a conflict between the US and Turkey would mean for the rest of NATO: it certainly would set a precedent, as never before has fighting broken out between two alliance members.