Early deployments of US special forces in Syria were intended to support the Kurdish YPG in fighting against ISIS, and while the continued increase of ground troops is officially presented as part of the ISIS war, a growing number of US troops find themselves deployed as “buffers” to try to prevent fighting between US allies.

The most recent US deployments have been to the northeastern border region of Syria, along the frontier between the Kurdish-held Hasakeh Province and neighboring Turkey, aimed at preventing the Turkish military from further attacks against the Kurdish YPG.

Turkey launch a flurry of airstrikes against Kurdish targets in Iraq and Syria a little over a week ago, with US officials expressing opposition to the attacks by their ally Turkey on their Kurdish allies. There have been smaller skirmishes since, though the US presence appears to have kept things from escalating.

This isn’t the first time international troops in Syria have been asked to play referee between two forces they are nominally supportive of, but who are in open warfare, as Russian troops repeatedly kept Turkey and its rebel auxiliaries away from the Syrian military early in the ceasefire.

The US first got involved in this effort by deploying a number of ground troops into the city of Manbij. The city is held by the Kurdish YPG, and Turkey has promised to attack and capture it, though US troops stationed there have so far prevented any attempted invasions.

The deployments are largely a gamble that NATO member Turkey won’t risk causing US military casualties with an aggressive operation, though President Erdogan has complained they amount to the US “supporting terrorism,” and has vowed to bring the matter up in an upcoming visit to the White House.