The book explores the reasons why the Second World War broke out in September 1939 and not sooner, and why a European war expanded into world war by 1941. The war has usually been seen simply as Hitler's war and yet the wider conflict that broke out when Germany invaded Poland was not the war that Hitler wanted. He had hoped for a short war against Poland; instead, Britain and France declared war on Germany.

Richard Overy argues that any explanation of the outbreak of hostilities must therefore be multi-national and he shows how the war's origins are to be found in the basic instability of the international system that was brought about by the decline of the old empires of Britain and France and the rise of ambitious new powers, Italy, Germany and Japan, keen to build new empires of their own.