The rise of the Islamic State

From the BBC

The rapid advance across Iraq by militant fighters from the Islamic State, widely known as Isis, has thrown the country into chaos and is threatening to redefine the Middle East.

The brutal, extremist group, which claims to have fighters from across the world, announced the creation of a "caliphate" - an Islamic state - across its claimed territory in Iraq and Syria a month ago. How did the Islamic State (IS) become such a powerful force?'

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