It's hard to believe World War I was once called The Great War, when an even greater war followed it.
The two World Wars are taken up as a single subject in Ian Kershaw's history book To Hell and Back: Europe 1914-1949.
The book is the latest in the Penguin "History of Europe" series, a tracing of causes and players through one of the most wrenching periods in human history.
Ian Kershaw joins us with details of his work.