Iraq conflict: What is a 'caliphate'?

Islamist militant group Isis has said it is establishing a caliphate, or Islamic state, on the territories it controls in Iraq and Syria.

Setting up a caliphate ruled by the strict Islamic law has long been a goal of many jihadists.

Fawaz Gerges, professor of international relations at the London School of Economics, provides context to the term 'caliphate'.

First broadcast on BBC Radio 4's Today programme on Monday 30 June.