This is a natural choice to start because after Japan had attacked Pearl Harbour the United States joined the war and practically allied with Great Britain. Study ends On the 6th June 1944 the operation Overlord or the Normandy invasion started. The Normandy invasion is a famous military operation. However, the preparation time before the 6th June 1944, invasion and particularly the co-operation during the preparations are less known. There is a plethora of studies dealing with the co-operation but the sources of those studies are limited.
Some studies have also aged, so a new fresh analysis is needed.
These two great allies did not find a real harmony in the preparations. Roosevelt was slow to act when the war events would have needed quick decisions. His style to arrange only few meetings with his most important soldier, General George C. Marshall, rendered the whole work of the United States military leadership. Churchill's imagination and quickly changing opinions frustrated his key man General Alan Brooke who, however usually managed to keep Churchill on line. One of the main problem in this co-operation was the lack of personal contact between Marshall and Brooke.