Abstract This study offers a new perspective on the publicly well-known event Normandy invasion. However, this study concentrates on the preparation time of the invasion. The subject is co-operation and it is done by careful investigation of original sources. The minutes of American and British meetings and conferences can be regarded as the foundation stones of this research. The study covers the time December 1941 to the sixth June 1944. 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. The co-operation between the American and British war leaders can be described as a mixture of distrust, selfishness - especially on making decisions of strategy - and subjectivity. 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. The problems were solved many times by Field-marshal John Dill - the leader of British Joint Staff Mission in Washington. Dill reached the trust of every single person in the alliance. Harry Hopkins as president Roosevelt's close personal adviser was a great link between the president and Marshall. The battle of Stalingrad changed the position of view on the Normandy invasion planning. After Stalingrad Roosevelt thought the French invasion as a likely cornerstone of the American politics after the war. By Overlord the power of Soviet Union could at least be limited. General Dwight David Eisenhower as the Supreme Commander of Overlord operation lead the final preparations in spring 1944. Eisenhower had on several occasions a narrow margin to act because both political and military pressure. In spite of this he was successful and like Hopkins and Dill he was the main person to ensure that the co-operation worked.