During the last months of peace in 1939, Eden joined the Territorial Army with the rank of major, in the London Rangers motorised battalion of the King's Royal Rifle Corps and was at annual camp with them in Beaulieu, Hampshire, when he heard news of the Molotov–Ribbentrop Pact.

On the outbreak of war, on 3 September 1939, Eden, unlike most Territorials, did not mobilise for active service.
Instead, he returned to Chamberlain's government as Secretary of State for Dominion Affairs and he visited Palestine in February 1940 to inspect the Second Australian Imperial Force.
However, he was not in the War Cabinet.
As a result, he was not a candidate for prime minister when Chamberlain resigned in May 1940 after the Narvik Debate and Churchill became prime minister.
Churchill appointed Eden Secretary of State for War.

At the end of 1940, Eden returned to the Foreign Office and became a member of the executive committee of the Political Warfare Executive in 1941.
Although he was one of Churchill's closest confidants, his role in wartime was restricted because Churchill conducted himself the most important negotiations, with Franklin D. Roosevelt and Joseph Stalin, but Eden served loyally as Churchill's lieutenant.
In December 1941, he travelled by ship to Russia where he met the Soviet leader Stalin  and surveyed the battlefields upon which the Soviets had successfully defended Moscow from the German Army attack in Operation Barbarossa.

Nevertheless, he was in charge of handling most of the relations between Britain and the Free French leader, Charles de Gaulle, during the last years of the war.
Eden was often both critical of the emphasis Churchill put on the special relationship with the United States and disappointed by the American treatment of its British allies.

In 1942, Eden was given the additional role of Leader of the House of Commons.
He was considered for various other major jobs during and after the war, including Commander-in-Chief Middle East in 1942 (which would have been a very unusual appointment as Eden was a civilian; General Harold Alexander would be appointed), Viceroy of India in 1943 (General Archibald Wavell was appointed to this job) or Secretary-General of the newly-formed United Nations Organisation in 1945.
In 1943, with the revelation of the Katyn Massacre, Eden refused to help the Polish Government in Exile.
Eden supported the idea of postwar expulsion of ethnic Germans from Czechoslovakia.

In early 1943, Eden blocked a request from the Bulgarian authorities to aid with deporting part of the Jewish population from newly-acquired Bulgarian territories to British-controlled Palestine.
After his refusal, some of the people were transported to German concentration camps in Nazi-occupied Poland.

In 1944, Eden went to Moscow to negotiate with the Soviet Union at the Tolstoy Conference.
Eden also opposed the Morgenthau Plan to deindustrialise Germany.
After the Stalag Luft III murders, he vowed in the House of Commons to bring the perpetrators of the crime to "exemplary justice", which led to a successful manhunt after the war by the Royal Air Force's Special Investigation Branch.

Eden's eldest son, Pilot Officer Simon Gascoigne Eden, went missing in action and was later declared dead; he was serving as a navigator with the Royal Air Force in Burma in June 1945.
There was a close bond between Eden and Simon, and Simon's death was a great personal shock to his father.
Mrs Eden reportedly reacted to her son's loss differently, which led to a breakdown in the marriage.
De Gaulle wrote him a personal letter of condolence in French.

In 1945, he was mentioned by Halvdan Koht among seven candidates who were qualified for the Nobel Prize in Peace.
However, he did not explicitly nominate any of them.
The person who was actually nominated was Cordell Hull.
During the last months of peace in 1939, Eden joined the Territorial Army with the rank of major, in the London Rangers motorised battalion of the King's Royal Rifle Corps and was at annual camp with them in Beaulieu, Hampshire, when he heard news of the Molotov–Ribbentrop Pact.
On the outbreak of war, on 3 September 1939, Eden, unlike most Territorials, did not mobilise for active service. Instead, he returned to Chamberlain's government as Secretary of State for Dominion Affairs and he visited Palestine in February 1940 to inspect the Second Australian Imperial Force. However, he was not in the War Cabinet. As a result, he was not a candidate for prime minister when Chamberlain resigned in May 1940 after the Narvik Debate and Churchill became prime minister. Churchill appointed Eden Secretary of State for War.
At the end of 1940, Eden returned to the Foreign Office and became a member of the executive committee of the Political Warfare Executive in 1941. Although he was one of Churchill's closest confidants, his role in wartime was restricted because Churchill conducted himself the most important negotiations, with Franklin D. Roosevelt and Joseph Stalin, but Eden served loyally as Churchill's lieutenant. In December 1941, he travelled by ship to Russia where he met the Soviet leader Stalin  and surveyed the battlefields upon which the Soviets had successfully defended Moscow from the German Army attack in Operation Barbarossa.
Nevertheless, he was in charge of handling most of the relations between Britain and the Free French leader, Charles de Gaulle, during the last years of the war. Eden was often both critical of the emphasis Churchill put on the special relationship with the United States and disappointed by the American treatment of its British allies.
In 1942, Eden was given the additional role of Leader of the House of Commons. He was considered for various other major jobs during and after the war, including Commander-in-Chief Middle East in 1942 (which would have been a very unusual appointment as Eden was a civilian; General Harold Alexander would be appointed), Viceroy of India in 1943 (General Archibald Wavell was appointed to this job) or Secretary-General of the newly-formed United Nations Organisation in 1945. In 1943, with the revelation of the Katyn Massacre, Eden refused to help the Polish Government in Exile. Eden supported the idea of postwar expulsion of ethnic Germans from Czechoslovakia.
In early 1943, Eden blocked a request from the Bulgarian authorities to aid with deporting part of the Jewish population from newly-acquired Bulgarian territories to British-controlled Palestine. After his refusal, some of the people were transported to German concentration camps in Nazi-occupied Poland.
In 1944, Eden went to Moscow to negotiate with the Soviet Union at the Tolstoy Conference. Eden also opposed the Morgenthau Plan to deindustrialise Germany. After the Stalag Luft III murders, he vowed in the House of Commons to bring the perpetrators of the crime to "exemplary justice", which led to a successful manhunt after the war by the Royal Air Force's Special Investigation Branch.
Eden's eldest son, Pilot Officer Simon Gascoigne Eden, went missing in action and was later declared dead; he was serving as a navigator with the Royal Air Force in Burma in June 1945. There was a close bond between Eden and Simon, and Simon's death was a great personal shock to his father. Mrs Eden reportedly reacted to her son's loss differently, which led to a breakdown in the marriage. De Gaulle wrote him a personal letter of condolence in French.
In 1945, he was mentioned by Halvdan Koht among seven candidates who were qualified for the Nobel Prize in Peace. However, he did not explicitly nominate any of them. The person who was actually nominated was Cordell Hull.
Anthony Eden