What is the significance of the battle for Mosul?

In June 2014 the Islamist group which calls itself the Islamic State in Iraq and the Levant captured Iraq's second city, Mosul.

Later that month the group announced it was establishing a 'caliphate', or an Islamic state, on the territories it controlled in Iraq and Syria.

But it has been losing much of the ground in gained in Iraq and 30,000 Iraqi troops began the assault to retake the city. So what is the significance of this major military push and what will be the outcome?

For the Sunday Politics, Adam Fleming reports.

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