(L-R) General Henri Giraud, Franklin Delano Roosevelt, Charles de Gaulle, and Winston Churchill at the Casablanca Conference in 1943
Up until the end of 1941, the Second World War was mainly a European War. By June 1940, Hitler had invaded most of Western Europe. At this point only Great Britain remained unconquered and still engaged in fighting Nazi Germany, bolstered by a significant Royal Navy, Royal Air Force and the English Channel, which made invasion difficult.
Britain’s Winston Churchill realised that the war could not be won without the help of the United States - but there was a problem. After the First World War most Americans were against getting involved in further international wars. During the late 1930s, Franklin D. Roosevelt, then president, had signed into law Neutrality Acts requiring the United States to avoid such wars. However, Churchill was a good lobbyist and Roosevelt was sympathetic to supporting Great Britain.
As a result, the “Lend-Lease Policy” came into being in which the United States supplied war materials, food and other supplies to Great Britain paid for by loans to Britain and the lease of British foreign bases. This policy was also extended to many other countries including the Free French.
In June 1941, Hitler invaded the USSR, opening a whole new Eastern Front to the war. Then in December 1941, Japan attacked the United States at Pearl Harbour. This changed American public opinion on the war overnight and simultaneously, Hitler and the axis forces declared war on the United States.
Even before the United States joined the war it had agreed with Great Britain that a “European (Germany) First” policy should be employed. This meant that in all operations the defeat of Nazi Germany would take priority over the war with Japan. This made sense as there was real concern in 1941 that Hitler could defeat the USSR. This would then allow him to invade Britain, leaving the United States to defeat Nazi Germany without its key ally.
Many countries sided with the Allies, including British Commonwealth countries such as Australia, Canada, New Zealand and South Africa. In addition, a number of the countries which had been invaded, including France, had governments in exile who were in the Allied camp.
A key factor in the Allies’ success was how well they worked together, no doubt facilitated by leadership from the top - Roosevelt and Churchill. In wartime there are always differences of opinion concerning strategy and tactics, not to mention the egos of the various generals. However, the invasion of Normandy was a clear example of Allied leadership, with armies working together to agree goals, resolve differences, decide who was in charge of what and produce a strategy with timelines that everyone worked to.