This piece from 2015 is long, but the first section has a really good explanation of why ISIS started operating in Syria, which of course is how we ended up there, and the forms that they’ve taken on the ground.
Jihadi fighters in Iraq and Syria reveal the apocalyptic motivations of the militant movement that has hijacked the Syrian uprising – and transformed the Middle East
For more than a century, Dabiq was one of northern Syria’s forsaken villages, a speck on a vast agricultural plain between the Turkish border and the deserts of Iraq, which hardly seemed likely to shape the fate of nations. A weathered sign at its entrance said 4,000 people lived there, most of whom appeared to have left by 2013, driven out over time by a lack of work – and lately by insurrection. For the first three years of Syria’s civil war, the arrival of a strange car would lure bored children to the town’s otherwise empty streets, scattering cats and chickens as they scampered after it. Little else moved.
Dabiq’s few remaining men worked on the odd building project: a half-finished mosque, a humble house for one local who had just returned after 10 years labouring in Lebanon, or a fence for the shrine that was the town’s only showpiece – the tomb of Sulayman ibn Abd al-Malik. The Ummayad caliph was buried under a mound of earth in 717, which over many centuries had somehow grown into a small hill. The war was happening elsewhere, it seemed.
Martin Churlov on why Isis fights – audio long read
That was until the jihadists of Islamic State (Isis) arrived in early 2014, an event that the Dabiq elders had feared from the moment the war began – and which the new arrivals had anticipated for much longer. To the foreigners, and the leaders of the new militant juggernaut who were beckoning them, the war had by then entered a new phase that would transform the tussle for power in Syria into something far more grand and important. For them, the conflict that was slicing the country apart was not merely, as the Syrian opposition had seen it, a modern struggle between a ruthless state and a restive underclass. The jihadis instead saw themselves at the vanguard of a war that many among them believed had been preordained in the formative days of Islam.