Tal Saman, Syria — “Raqqa we are coming” say the words spray-painted in Kurdish at the entrance to this empty little town, which lies on the front line of a U.S.-backed advance toward the Islamic State’s capital.

The city of Raqqa is 17 miles away, a tantalizingly short hop to the place showcased in the militants’ propaganda videos as an Islamist utopia, where the terrorist attacks in Paris and Brussels were planned and where, U.S. officials warn, new plots against the West are being forged.

But a full offensive to retake the city could still be months or more away, despite hopes in Washington that an operation to take the Islamic State’s most symbolically significant stronghold would be well underway before President Obama left office.

A rare visit to the Raqqa front line illustrated how near and yet so far the defeat of the Islamic State remains. The battle for Mosul in neighboring Iraq has stalled, the attack in Berlin has brought home the continued threat of terrorism, and there is still no plan for an offensive on Raqqa, making the war one of the most immediate, and complicated, challenges the Trump administration will have to confront.

Meanwhile, a preliminary operation to isolate and besiege Raqqa is going well. Over the past month, a Kurdish-Arab alliance called the Syrian Democratic Forces has been slicing briskly through Islamic State lines in the northern and western countryside of Raqqa province. The fighters have captured some 140 villages and nearly 800 square miles of mostly empty rural land on two fronts in a just over a month, countering little resistance along the way.

This is not the battle for Mosul, where large armored formations are converging from different directions. There are more sheep than soldiers scattered across the empty fields. Flocks trot through the landscape herded by boys on donkeys as the lightly armored pickup trucks and SUVs used by the Kurdish and Arab militia weave among them.

Every now and then, American soldiers hurtle past, a reminder that the U.S. military is very much invested in the Raqqa front, however remote it may be. There are around 600 Special Operations troops embedded with the SDF in northeastern Syria, a number that could rise before the battle fully takes shape, U.S. officials say. One of those troops was killed on Nov. 24, the first U.S. casualty of the war in Syria.

He died in Tal Saman, a victim of one of the mines and booby traps that have become the Islamic State’s hallmark defense against advancing foes in Iraq and Syria.

Otherwise, the militants have put up little resistance, firing mortars as the soldiers advance but retreating well before their enemies arrive.

Bigger obstacles loom, however, in the form of a geopolitical tangle that could prove more daunting than any defenses mounted by the Islamic State.

At the heart of the issue is the U.S. military’s policy of sending arms to the area controlled by the main Syrian Kurdish militia, the People’s Protection Units, widely known as the YPG.

The decision has paid off so far. The YPG — which constitutes the Kurdish component of the SDF — has proved to be the United States’ most effective military ally in Syria, and it has retaken vast swaths of territory.

It is also expanding deep into Arab areas as it presses forward against the militants, raising questions among observers about the long-term sustainability of the gains.