The Islamic State of Iraq and Syria, which has claimed responsibility for Friday's terror attacks in Paris, has its origins in Iraq, but the group as we know it today is in many ways a product of Syria's civil war. That war is much bigger than ISIS, but it is crucial for understanding so much that has happened in the past year, from terror attacks to the refugee crisis. And to understand the war, you need to understand how it began and how it unfolded:
ISIS, you will note, did not become a major player in the Syrian war until after some time — and after the country had been torn apart by a conflict that initially had nothing to do with the group. The fight at the center of the war, between Syrian leader Bashar al-Assad and the opposition, is thus a big part of how ISIS can exist and could take so much territory.
Something else important you'll notice: As more outside groups get involved in the war, each escalates by backing their side, then a rival will also get involved to back the other side. So what you have is not just the Syrian factions escalating, but regional and global powers escalating as well, thus worsening the war and entrenching Syria's divisions.
You'll also see that the countries getting involved don't always have the same objectives, and can end up working against even their allies. When Gulf states begin funding Syria's rebels, for example, they are mostly seeking to topple Assad and set back his patron, Iran, so they often fund extremists, believing they're better fighters. And different Gulf states fund different groups — at first Qatar is the most active, then Saudi Arabia — that are sometimes at odds with one another. And so on. These internal contradictions are an important part of understand how the war has gotten so bad.
For more, read our brief history of Syria's war, from the rise of the opposition to the refugee crisis to why the US and Russia ended up intervening.