The Islamic State in Iraq and Syria, the militant group more frequently known as ISIS, took the world by surprise when it swept through northern Iraq in June 2014. ISIS then attempted to set up a kind of hyper-violent, extremist state in eastern Syria and northern Iraq — believing, in their minds, that they might bring on the apocalypse.
Since 2014, ISIS has been on the decline — losing control over the vast bulk of its territorial empire due to concerted military pushback on the ground and a US-led international intervention. But the group remains a threat, and may yet resurge and once again threaten the stability of northern Iraq.
What follows is a guide to the the 18 key facts you need to understand ISIS, what it was like when it controlled vast swaths of territory, and the key reasons why its empire dwindled without the group ending.