The diplomatic history of World War II includes the major foreign policies and interactions inside the opposing coalitions, the Allies of World War II and the Axis powers, between 1939 and 1945.
High-level diplomacy began as soon as the war started in 1939. The British Prime Minister, Winston Churchill, forged close ties with France and sought close ties with the United States, especially through his relationship with President Franklin D. Roosevelt. Following the Soviet Union's entry into the war in June 1941, Churchill called the association of the United Kingdom, the United States, the Soviet Union, and other Allies the "Grand Alliance". The United States also saw China and its leader Chiang Kai-shek as its main ally in Asia and considered it one of the "Big Four" allied powers. American diplomacy stepped up after it entered the war in December 1941 and was bolstered by large quantities of financial and economic assistance, especially after the Lend-Lease programme began to attain full strength during 1943. The Soviet Union's main diplomatic goal at first was to win support to defend against the massive German invasion. With victory in sight by 1944, Moscow began creating satellite states, first of all in Poland and East Germany. The main British goals were to defeat the German threat, maintain British roles in Central and Eastern Europe, and preserve the British Empire. The British dominions and India contributed significantly to warfighting but did not have a voice in major Allied decisions. Roosevelt was hostile to the idea of the British, French, and other empires, but was forced by Churchill to postpone interference in India. Roosevelt's main goal by 1943 was the creation of a postwar United Nations, controlled by the Allied Big Four — the Soviet Union, China, the United Kingdom, and the United States — with major roles also for France. Roosevelt was increasingly troubled by Moscow's aggressive intentions late in the war but decided that with the United Nations in place, and his own persuasive personal relationship with Stalin, problems could be resolved after the war.
For the Axis powers diplomacy was a minor factor. The alliance of Germany, Italy, and Japan was always informal, with minimal assistance or coordination. Hitler had full control of German diplomatic policies and imposed his will on his allies in Eastern Europe, and with the puppet regime in northern Italy after 1943. Japan's diplomats had a minor role in the war, as the military was in full control. A dramatic failure was the inability of Tokyo to obtain the formulas for synthetic oil from Germany until it was too late to overcome the fatal shortage of fuel for the Japanese war machine. Practically all the neutral countries broke with Germany before the end of the war and thereby were enabled to join the new United Nations.
The military history of the war is covered at World War II. The prewar diplomacy is covered in Causes of World War II and International relations (1919–1939). After the war, diplomacy revolved around the Cold War.
Allies
The Allies of World War II began to form in September 1939 when Poland was invaded and Britain and France declared war on Nazi Germany. Except for Ireland, which remained neutral throughout the war, the Commonwealth Dominions (Canada, Australia, New Zealand, and South Africa) all declared war alongside Great Britain but no other nations joined their cause. The Anglo-French Supreme War Council (SWC), established at the outset, coordinated a joint military strategy. It operated until the Battle of France concluded with a successful German invasion in June 1940, after which France surrendered and Britain and its Empire continued the war against Germany.
The First Inter-Allied Conference took place in London in early June 1941 between the United Kingdom, the four co-belligerent British Dominions (Canada, Australia, New Zealand and South Africa), the eight governments in exile (Belgium, Czechoslovakia, Greece, Luxembourg, the Netherlands, Norway, Poland, Yugoslavia) and Free France.
The United States developed a secret plan, should it enter the war, to coordinate military objectives and action with the British at a military staff conference in early 1941. Through the Lend-Lease Act in March 1941, the US moved away from a position of neutrality and non-interventionism, adopting a policy to provide war materiel to the Allies. The Soviet Union, having first cooperated with Germany in invading Poland whilst remaining neutral in the Allied-Axis conflict, entered an alliance with Britain in July 1941 after being invaded by Germany. At the Atlantic Conference in August, the U.S. and Britain proposed a Soviet aid conference and set out principles for the post-war world in the Atlantic Charter, to which the other Allies, now including the Soviet Union, agreed to adhere in September. After the Japanese attack on Pearl Harbor, the U.S. joined the war in December 1941. China had already been in a prolonged war with Japan since the Marco Polo Bridge incident of 1937, and joined the Allies during December 1941.
The Grand Alliance The United States, the United Kingdom, and the Soviet Union formed the "Big Three" Allied powers. The Big Three and China were recognized as the "Big Four", acknowledging their central role in prosecuting the war.Hoopes, Townsend, and Douglas Brinkley. FDR and the Creation of the U.N. (Yale University Press, 1997). They were in frequent contact through ambassadors, top generals, foreign ministers and special emissaries such as the American Harry Hopkins. Relations between the four resulted in the major decisions that shaped the war effort and planned for the postwar world. Cooperation between the United Kingdom and the United States was especially close and included forming a Combined Chiefs of Staff.
There were numerous high-level conferences; Churchill attended 14 meetings, Roosevelt 12, and Stalin 5. Most visible were the three summit conferences that brought together the three top leaders.Herbert Feis, Churchill Roosevelt Stalin: The War They Waged and the Peace They Sought: A Diplomatic History of World War II (1957)William Hardy McNeill, America, Britain and Russia: their co-operation and conflict, 1941–1946 (1953) The Allied policy toward Germany and Japan evolved and developed at these three conferences.
Europe first
At the December 1941 Arcadia Conference US President Franklin Roosevelt and British Prime Minister Winston Churchill met in Washington, shortly after the United States entered the war.John F. Shortal, Code Name Arcadia: The First Wartime Conference of Churchill and Roosevelt (Texas A&M University Press, 2021). They agreed on the key elements of the grand strategy. The United States and the United Kingdom would use the preponderance of their resources to subdue Nazi Germany in Europe first. Simultaneously, they would fight a holding action and limited offensive against Japan in the Pacific, using fewer resources. After the defeat of Germany—considered the greatest threat to the UK and the Soviet Union—all Allied forces could be concentrated against Japan.
The Europe first policy remained in effect throughout the war, however, the terms "holding action" and "limited offensive" in the Pacific War were subject to interpretation and modification at Allied leaders' conferences and by US senior military commanders. At Arcadia, the US agreed to send bombers to bases in England and the British agreed to strengthen their forces in the Pacific. The British rejected American proposals for a "suicidal" invasion of northern Europe in 1942. Churchill pressed instead for a landing in French North Africa during 1942. With Roosevelt's support, in July 1942 Operation Torch was scheduled for later that year. Nonetheless, it was the strategic situation in the Pacific and related logistical requirements that dominated the United States' actions after it entered into the war and led to an initial focus on the Pacific. By 1944 and 1945, the balance of US resources shifted heavily towards the European Theatre as Europe's first strategy became a reality rather than just a stated objective. Even in these later stages of the war, there was intense competition for resources as operations in both regions were scaled up.
Tehran Conference
Following preparation at the Moscow Conference in October–November 1943, the first meeting of the Big Three, Stalin, Roosevelt, and Churchill, came at the Tehran Conference in Iran from 28 November to 1 December 1943. It agreed on an invasion of France in 1944 (the "Second Front") and dealt with Turkey, Iran, the provisional Yugoslavia, and the war against Japan as well as the postwar settlement.
Yalta Conference
The Yalta Conference met in the Crimea 4–11 February 1945. It focused on postwar plans for European boundaries. The Soviets already controlled Poland. The new boundaries shifted Poland westward. Stalin was promised control of western Belorussia and western Ukraine. Poland was to gain parts of Germany. Stalin promised free elections in Poland under the auspices of a government he controlled. At Roosevelt's strong urging, Stalin agreed to enter the war against Japan three months after the defeat of Germany. It was agreed the USSR would be a member of the United Nations Security Council, with a veto, and Ukraine and Belorussia would be UN members, but not the other 12 Soviet republics. Germany was to be divided into three zones of occupation, and France was also to get a zone. In a decision that became highly controversial, all civilians would be repatriated.Fraser J. Harbutt, Yalta 1945: Europe and America at the Crossroads (2010).
Potsdam Conference
The Potsdam Conference was held from 17 July to 2 August 1945, at Potsdam, Germany, near Berlin. Stalin met with the new US President Harry S. Truman and two British prime ministers in succession—Winston Churchill and Clement Attlee. It demanded "unconditional surrender" from Japan, and finalized arrangements for Germany to be occupied and controlled by the Allied Control Commission. The status of other occupied countries was discussed in line with the basic agreements made earlier at Yalta.Herbert Feis, Between War and Peace: The Potsdam Conference (1960).
The United Nations
The Declaration by United Nations formalized the Allies in January 1942. The Big Four (the United States, United Kingdom, Soviet Union, and China) were joined by numerous other Allied countries who had signed the Declaration and declared war on the Axis powers. Under Roosevelt's leadership, this "United Nations" alliance in 1945 became a new organization to replace the defunct League of Nations.Townsend Hoopes, and Douglas Brinkley, FDR and the Creation of the UN (Yale UP, 1997).
Four Policemen
The Four Policemen was a council with the Big Four that U.S. president Franklin D. Roosevelt proposed as a guarantor of world peace. Their members were called the Four Powers during World War II and were the four major Allies of World War II: the United Kingdom, the United States, the Soviet Union, and the Republic of China. Roosevelt repeatedly used the term "Four Policemen" starting in 1942.Richard W. Van Alstyne, "The United States and Russia in World War II: Part I" Current History 19#111 (1950), pp. 257-260 online As a compromise with internationalist critics, the Big Four nations became the permanent members of the UN Security Council, with significantly less power than had been envisioned in the Four Policemen proposal. When the United Nations was officially established later in 1945, France was in due course added as the fifth permanent member of the Security Council because of the insistence of Churchill.
Moscow Conference of 1943