The Islamic State of Iraq and Syria has solidified control over large portions of both countries. But a map of the two countries showing the stretches held by ISIS, as well as contested by it, reveals a simple pattern: the Tigris and Euphrates. The two rivers meet in Baghdad, of course, and that is where the largest contested areas currently happen to be. Their river basins go all the way to the Taurus Mountains of eastern Turkey. To the east, they reach deep into Iran. Interestingly, all the portions held by ISIS are linear. Hugging water all the time, they occasionally stretch to the most important tributaries of the two rivers. One way or another, the militants building a new caliphate are pretty dependent on water. One look at the map of the region demonstrates this fact beyond any doubt. It could well be called the Tigris and Euphrates Caliphate. Chances are that Greek and Roman generals of a couple of millennia ago would figure out Caliph Ibrahim in a jiffy.