The remarkable success of Islamic State of Iraq and Syria (ISIS) in capturing huge swaths of land in Iraq and Syria is even more surprising when you consider the scale of the lands captured: Iraq and Syria are big countries, and ISIS controls a lot of them.
This map helps to put ISIS' reach into perspective for U.S. readers. As you can see, it pretty much stretches from Illinois to Virginia.
There's another remarkable factor here. ISIS troop numbers are not huge: According to The Guardian, just 800 militants managed to force 30,000 Iraqi soldiers to flee in Mosul. In total, there are believed to be between 7,000 and 10,000 fighters.
ISIS is only able to hold this much land with significant support from the local Sunni population, as well as the help of Baathist loyalists and other armed groups.