Book excerpt: As the war in Europe entered its final months, we teetered on the edge of a Third World War. While Soviet forces smashed their way into Berlin, Churchill ordered British military planners to prepare the top-secret Operation Unthinkable - the plan for an Allied attack on the Soviet Union - on 1 July 1945. The plan called for the use of the atomic bomb and Nazi troops if necessary: more than merely controversial, as the extent of the Holocaust was becoming clear.A haunting study of the war that so nearly was, Walker offers a fascinating insight into the upheaval as the Second World War drew to a close and the Allies' mistrust of the Soviet Union that would blossom into the Cold War.