In the mid-twentieth century the relationship between America and Britain had a chequered past. Theirs was a history of protection and oppression, of rebellion and ultimately war. But then the shared crisis of the Second World War brought Britain and America closer than ever before or since, and saw an unprecedented level of military cooperation. How was such a radical shift possible?

To uncover how this historically fraught relationship recovered from its inauspicious start, Niall Barr goes back to the origins of their shared military history in the American War of Independence and shows how these early days had ramifications for the later crucial alliance.

Picking up the tale with America's entry into the Second World War, Niall Barr tells the story of these two armies as they fought in the largest war in history, from the uppermost echelons of the relationship between Churchill and Roosevelt right down to battlefield level and the soldiers fighting side by side for a common cause.