“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.