How has ISIS become one of the richest ever militant groups?

Editor’s Note: Over the last few weeks the Islamic State in Iraq and Syria (ISIS) has expanded from its stronghold in northern Syria across large swathes of western Iraq. The speed and scale of the advance has caught most observers by surprise, as have reports that put the jihadist group’s wealth as high as $2 billion. But just where does ISIS get the mountains of money that make it such a potent force? Click on the flashing icons above to find out.