Islamic State's stunning success this summer as it swept across northern Iraq and Syria flows from a highly organized structure controlled by a tightknit cadre led by an Islamist zealot who learned from the mistakes of his al Qaeda predecessors.

Blending familiar terrorist acts such as car bombings with conventional military tactics, the group bolsters its strength with local tribal connections and the skills of former generals in Saddam Hussein's army, said Western and Middle Eastern officials tracking the extremist movement.

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