Documentary. The inside story of how a small band of fanatical jihadists became the world's richest terror army. Peter Taylor looks at the wealth of the so-called Islamic State.

The inside story of how a small band of fanatical jihadi fighters became the world's richest terror army ever. Featuring the first major TV interview with an imprisoned senior leader of the so-called Islamic State, Peter Taylor looks behind its medieval savagery and investigates how it became so fabulously rich and resilient. Part of a season of films on BBC Two about the Islamic State.