It is arguably the most important year in modern history. With the British Expeditionary Force trapped at Dunkirk, Prime Minister Neville Chamberlain replaced by the maverick Winston Churchill and France collapsing under the force of invasion, the world watched and waited to see which way the axis of power would tilt. James Holland, leading historian of the period, joins his brother Tom and Dominic Sandbrook to discuss the events of 1940.