The United States military is getting drawn into a deepening struggle for control over areas liberated from Isis that risks prolonging American involvement in wars in Syria and Iraq long after the militants are defeated.
In their first diversion from the task of fighting Isis (Islamic State) since the US military's involvement began in 2014, US troops dispatched to Syria have headed in recent days to the northern town of Manbij, 135km northwest of the extremists' capital, Raqqa, to protect their Kurdish and Arab allies against a threatened assault by other US allies in a Turkish backed force.
Russian troops have also shown up in Manbij under a separate deal that was negotiated without the input of the US, according to US officials.
Under the deal, Syrian troops are to be deployed in the area, also in some form of peacekeeping role, setting up what is effectively a scramble by the armies of four nations to carve up a collection of mostly empty villages in a remote corner of Syria.
The latest twist in Syria's ever more complicated war points to one of the many risks of a US strategy that has prioritised the military defeat of the Isis at the expense of political solutions to the broader conflicts fuelling instability in the wider region, analysts say.
Other wars are brewing elsewhere across the vast areas freed so far from Isis control, in Iraq as well as Syria. In recent days, the US has been mediating between rival Kurdish factions in Iraq, both of them indirectly allied to the US in the fight against Isis, after clashes erupted around the northwestern town of Snune, freed from Isis more than two years ago.
Manbij is the first instance, however, in which US troops have become directly involved in keeping rival factions apart.
The Pentagon has described their mission as one to "reassure and deter" local parties from attacking one another, a new role for the US military in the fight against Isis.
The deployment is "fraught with risk", said Robert Ford, who served as the Obama Administration's last ambassador to Syria until 2014. He is now with the Washington-based Middle East Institute and teaches at Yale University.
"That's not a small policy change. It is a huge policy change," he said. "We have never in our Syrian policy ever put US personnel in between warring Syrian factions or to maintain a local ceasefire."
Aaron Stein of the Washington-based Atlantic Council said the Manbij confusion points to the contradictions of a strategy that has focused on arming local factions to take on Isis without regard to existing rivalries.
Meanwhile, US Marines from an amphibious task force have left their ships in the Middle East and deployed to Syria, establishing an outpost from which they can fire artillery guns in support of the fight to take back the city of Raqqa from Isis.
The deployment marks another escalation in the US involvement in the war in Syria, and puts conventional US troops in the battle there.
Several hundred Special Operations troops have advised local forces there for months, but the Pentagon has shied away from using conventional forces in Syria.
The force is part of the 11th Marine Expeditionary Unit. The Marines on the ground include part of an artillery battery that can fire powerful 155mm shells from M777 Howitzers, two officials said.
The expeditionary unit's ground force, Battalion Landing Team 1st Battalion, 4th Marines, will man the guns and deliver fire support for US-backed local forces who are preparing an assault on the city.
Additional infantrymen from the unit are likely to provide security.
The Marine mission has similarities to an operation the Marine Corps undertook about a year ago when the US military was preparing to support an assault on the Iraqi city of Mosul.
In that case, a force from the 26th Marine Expeditionary Unit, of Camp Lejeune, North Carolina, established a fire base south of the city in support of Iraqi and Kurdish troops who were then carrying out operations to isolate Mosul from Isis-held territory around it.
The existence of the outpost near Mosul became public after it was attacked by rockets last March.