"It's extraordinary that Britain can't win the First World War without American help, I always think.

"And quite how the richest empire in the world fails to defeat the central powers unassisted is a profound mystery because one would have thought on any economic analysis that Britain had the resources in 1914 to do it without American help, but it turns out not to be the case. And that moment, when Britain realizes that it needs the United States to bail it out, and indeed, to come into the war to ensure victory, is one of the key turning points in world history because it represents the beginning of British decline."