How does ISIS fund its reign of terror?

The Islamic State's staggering successes come at a cost. After all, it's not cheap to wage war and manage territorial conquests whose population is now roughly the size of Austria's.

So how can ISIS, cut off from the rest of the world by financial and trade sanctions, and under daily aerial and land bombardment by some of the richest countries in the world, afford to maintain a well-armed military and pay other bills?

Interviews with Iraqi, Kurdish, European, Syrian and American government officials, analysts and intelligence agents sketch a portrait of ISIS's robust, sprawling, and efficient financial operation.

The terrorist group relies on a relatively complex system to manage its far-reaching networks.

Its currencies of choice -- cash, crude oil and contraband -- allow it to operate outside of legitimate banking channels.

Turkey's southern corridor, Iraq's northwestern corridor and Syria's northeastern corridor are key weak spots, well away from the prying eyes of outside investigators.

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