The Royal Collection is one of the most wide-ranging collections of art and artefacts in the world and provides an intriguing insight into the minds of the monarchs who assembled it. In this series, BBC Arts Editor Will Gompertz encounters dozens of these unique objects - some priceless, others no more than souvenirs - each shedding light on our relationship with the monarchy and giving a glimpse into the essential ingredients of a successful sovereign. In today's programme, Will puts the Royal Collection on a war footing as he examines six very different items covering 700 years that show how the monarchy has rallied their subjects for war and endeavoured to keep the peace. Will's explorations of monarchs at war begins with a photograph of the young Princess Elizabeth training to be an ATS Officer in 1945 shown to the nation to boost morale but then spools back to the Middle Ages with a clear symbol of martial authority - Edward III's six feet, eight inch long bearing sword. Henry VIII, Mary Queen of Scots and a elaborate object of apology from the city of Exeter all form part of the story of the monarchy in arms. As well as curators from the Royal Collection, Will meets General Sir Mike Jackson, Dr Duncan Anderson - Head of Academy, Sandhurst and Sir David Cannadine. Producer: Neil George.