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The British army in 2017 is in uncharted territory. They have not been at war for three years. After controversial campaigns in Iraq and Afghanistan, there is political and public opposition to military intervention overseas. The army's budgets are under pressure and they have the smallest troop numbers since the days of Cromwell. But now with the rise of the so-called Islamic State, the threat of a new cold war in Eastern Europe and famine and conflict in Africa, the British army have to play a new role in a deeply unstable world. Filmed over 18 months, this series goes into the heart of the British army. Through the eyes of the rank and file and the leaders, it shows the challenges of fighting wars, when we are not at war.

In this episode, the army returns to Iraq where they have a bloody history. 179 British soldiers were killed here between 2003 and 2011. Now they are helping the Iraqis defeat IS. As the battle for Mosul begins, the series is with the regiments operating behind the frontlines. Can the army face off their enemies, find lasting peace and avoid being drawn into costly new wars?

British soldiers from 1 Rifles are in northern Iraq. They have weeks to train a group of Kurdish recruits called the Green Eagles to fight IS. The Green Eagles are a part of the Kurdish Peshmerga. Many are civilians with little or no combat experience. It is a new role for the British army, who are used to doing the fighting themselves.

In 2014, IS swept across Iraq seizing a third of the country. An international coalition, including Britain, targeted their strongholds with air strikes weakening their grip. But IS still controls the city of Mosul, home to one and a half million people. To defeat IS, Mosul must be liberated. When IS captured Mosul, Kurdish recruit Ibrahim Dyab was a student. In the first days of war, two of his brothers and his father were killed. With the battle of Mosul underway, General Jones, the deputy commander of an international coalition of 73 countries and the most senior British army officer in Iraq, oversees the coalition's strategy to liberate Mosul. His job is to provide the Iraqi forces with tactical advice and use coalition air strikes to shape the battlefield. But in the first 51 days, IS launch over 200 suicide attacks in vehicles, known as VBIEDs. The attacks kill 2,700 Iraqi soldiers and destroy 25 per cent of the Iraqi army vehicles. Unlike past campaigns, General Jones can't put his coalition troops on the ground to solve the problem. He must convince the Iraqis to improve their military planning or else the battle for Mosul could fail.

Another regiment of the British army, 4 Rifles, are in Al Anbar province to protect a strategically important Iraqi air base at Al Asad. The region is a stronghold for IS. Before 4 Rifles arrived, Al Asad was under constant attack. The British army's role here is very different to past operations in Iraq. They can't leave the base or actively seek out the enemy. Hundreds of Iraqi army soldiers are stationed here. Their job is to cut the flow of IS fighters crossing the Syrian border 100 miles away and joining the fight in Mosul. The coalition has brought in heavy artillery and surveillance equipment to protect the Iraqis.

4 Rifles are also training soldiers from the Iraqi army here. In an area of widespread support for IS, this poses a higher level of threat than working with the Kurdish Peshmerga. 4 Rifles have intelligence that IS are trying to turn Iraqi soldiers against them. One of the Iraqi soldier's family members is being held by IS. They threatened to hurt them unless he shot a British solider. With a high risk of an insider attack, the 4 Rifles vigilantly watch Iraqi soldiers and civilian workers at Al Asad. As one British soldier says, they must be 'charming to every single person they meet but always have a plan to kill them'. The 4 Rifles are frustrated they cannot fight IS themselves. As neighbourhoods are liberated in East Mosul, tens of thousands of civilians flee to refugee camps. The Iraqi government's advice is to stay in their homes as long as it is safe to do so. This means civilians are not only at risk from collateral damage, but IS are holding thousands of them hostage and using them as human shields. They have executed thousands who try to escape. The Iraqi government's strategy is an uneasy compromise for General Jones, who worries the battle for Mosul could be undermined by a humanitarian crisis. It takes 94 days to liberate half of Mosul. The coalition's new role in the war has been controversial. Independent monitors claim 2,000 civilians have been killed in coalition airstrikes. For the British soldiers in Iraq, their new role is also not easy. There are no clean-cut victories like there were in the Falklands War. However, 2016 was the first time since 1968 that no British solider died in combat.