Experts say that ISIS is the largest terror organization in Iraq, placing the number of fighters in the thousands.
ISIS has presented itself as a champion of the Sunni cause, capitalizing on the rampant disenfranchisement many Sunnis feel under the Shiite-dominated government of Prime Minister Nouri al-Maliki.
That unhappiness, experts say, has given the group moderate support in parts of Iraq and also made victory in overtaking Sunni-dominated cities easier to achieve even while the militants are vastly outnumbered by Iraqi security forces.
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ISIS, though, is far from a "ragtag" army, according to Hayder al-Khoei, an Iraq analyst for London-based think tank Chatham House.
"They're more like a conventional army – rockets, missiles and the like," he explained.
The civil war in Syria has only helped ISIS fighters hone their craft – giving them battlefield skills plus staging and training grounds across the border that make the group even more dangerous.
By taking Mosul, the physical border with Syria becomes even less relevant for ISIS.
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"It's given them a stepping stone from the north," al-Khoei said. "Their territory is much more consolidated.. They control a lot of Syrian territory on the other side of the border so taking Mosul gives them more ability to maneuver."
In addition to his reputation for brutality, Baghdadi is particularly concerning to the West given his acceptance of scores of foreign fighters - who could one day take their skills back home to the U.S. or Europe and wage jihad.