After the Russian Revolution he was involved in the negotiations that took place to release British diplomats, including Robert Bruce Lockhart and Ernest Boyce, who had been arrested following a plot to overthrow Lenin and the Bolshevik government. On 2nd October, 1918, the British government arranged for the diplomats to be exchanged for captive Soviet officials such as Maxim Litvinov. This encouraged him to study Marxism: ""I had some vague impression of the revolutionary views of Lenin and Trotsky, but knew nothing of Marxism; I'd probably never heard of Marx".

The British government was divided over the issue of the Russian Civil War. Carr disagreed with the Minister of War, Winston Churchill, who took the controversial decision to use the stockpiles of M Device (diphenylaminechloroarsine) against the Red Army who were involved in fighting against invading forces hostile to the Russian Revolution. He was supported in this by Sir Keith Price, the head of the chemical warfare, at Porton Down. He declared it to be the "right medicine for the Bolshevist" and the terrain would enable it to "drift along very nicely". Price agreed with Churchill that the use of chemical weapons would lead to a rapid collapse of the Bolshevik government in Russia: "I believe if you got home only once with the Gas you would find no more Bolshies this side of Vologda."

In the greatest secrecy, 50,000 M Devices were shipped to Archangel, along with the weaponry required to fire them. Churchill sent a message to Major-General William Ironside: "Fullest use is now to be made of gas shell with your forces, or supplied by us to White Russian forces." He told Ironside that this "thermogenerator of arsenical dust that would penetrate all known types of protective mask". Churchill added that he would very much like the "Bolsheviks" to have it. Churchill also arranged for 10,000 respirators for the British troops and twenty-five specialist gas officers to use the equipment.

Some one leaked this information and Churchill was forced to answer questions on the subject in the House of Commons on 29th May 1919. Churchill insisted that it was the Red Army who was using chemical warfare: "I do not understand why, if they use poison gas, they should object to having it used against them. It is a very right and proper thing to employ poison gas against them." His statement was untrue. There is no evidence of Bolshevik forces using gas against British troops and it was Churchill himself who had authorised its initial use some six weeks earlier.

Carr argued that the Bolsheviks were destined to win the Russian Civil War, and that it made no political sense to be on the losing side. Major-General William Ironside told David Lloyd George that he was convinced that even after these gas attacks his troops would not be able to advance very far. He also warned that the White Army had experienced a series of mutinies (there were some in the British forces too). Lloyd George agreed that Ironside should withdraw his troops. This was completed by October, 1919. The remaining chemical weapons were considered to be too dangerous to be sent back to Britain and therefore it was decided to dump them into the White Sea.

In 1919, Carr was part of the British delegation at the Paris Peace Conference and was involved in the drafting of parts of the Treaty of Versailles relating to the League of Nations. Carr, like another young official at the conference, John Maynard Keynes, feared that the war reparations imposed on Germany could not be paid and this would led to further conflict in Europe. As Keynes pointed out: "The Treaty includes no provision for the economic rehabilitation of Europe - nothing to make the defeated Central Powers into good neighbours, nothing to stabilise the new States of Europe, nothing to reclaim Russia; nor does it promote in any way a compact of economic solidarity amongst the Allies themselves; no arrangement was reached at Paris for restoring the disordered finances of France and Italy, or to adjust the systems of the Old World and the New."

Carr married the widow, Anne Ward Howe, in January 1925, just before he took up an appointment at the British Embassy in Riga, Latvia, where he served as Second Secretary. A son, John, was born in 1926. During this period he learnt Russian and read a great deal of Russian literature. He was deeply influenced by the ideas of Alexander Herzen and Mikhail Bakunin. As Cathy Porter, the author of Fathers and Daughters: Russian Women in Revolution (1976) has pointed out: "In the 1830s writers like Belinsky, Bakunin, Herzen and Ogarev, all consumed by the desire for philosophical certainties, were tentatively exploring the ideas of socialism within a framework of romantic culture.... Herzen's quasi-religious desire for inner peace prompted him to mediate between the more extreme philosophies of his friends. On the other hand there was Bakunin, whose radical interpretation of the theories of Fourier, Saint-Simon and Owen were to lead him to a more doctrinaire violence." Carr wrote fifty years later: "The liberal moralistic ideology in which I was brought up was not, as I had always assumed, an Absolute taken for granted by the modern world, but was sharply and convincingly attacked by very intelligent people living outside the charmed circle, who looked at the world through very different eyes...This left me in a very confused state of mind: I reacted more and more sharply against the Western ideology, but still from a point within it".

Carr published a biography of Fyodor Dostoevsky in 1931. Using the pseudonym "John Hallett" Carr wrote several articles on Russia for journals such as The Spectator, The Fortnightly Review and the Times Literary Supplement. Carr bitterly attacked the government policy following the Russian Revolution: "It is not longer possible for any sane man to regard the campaigns of Kolchak, Yudenich, Denikin and Wrangel otherwise than as tragic blunders of colossal dimensions. They were monuments of folly in conception and of incompetence in execution; they cost, directly and indirectly, hundreds of thousands of lives; and except in so far as they may have increased the bitterness of the Soviet rulers against the "White" Russians and the Allies who half-heartedly supported them, they did not deflect the course of history by a single hair's breadth."

Carr, who had become concerned by the impact of the Great Depression, became increasingly supportive of the economic policies of Joseph Stalin. In February 1931 he wrote: "The Soviets... have discovered a new religion of the Kilowatt and the Machine, which may well be the creed for which modern civilization is waiting.... This new religion is growing up on the fringes of a Europe which has lost faith in herself. Contemporary Europe is aimlessly drifting, refusing to face unpalatable facts, and looking for external remedies for her difficulties. The important question for Europe at the present time is... whether the steel production of the Soviet Union will overtake that of Great Britain and France... whether Europe can discover in herself a driving force, an intensity of faith comparable to that now being generated in Russia".

Carr was concerned by Adolf Hitler coming to power in 1933 but he admired his attempts to solve Germany's economic problems: "The crucial point about Hitlerism is that its disciples not only believe in themselves, but believe in Germany. For the first time since the war a party appeared outside the narrow circles of the extreme Right which was not afraid to proclaim its pride in being German. It will perhaps one day be recognized as the greatest service of Hitlerism that, in a way quite unprecedented in German politics, it cut across all social distinctions, embracing in its ranks working men, bourgeoisie, intelligentsia and aristocrats."

Carr's views on Hitler brought him into conflict with his superior at the Foreign Office, the Permanent Undersecretary Sir Robert Vansittart, who wrote on 6th May 1933: "The present regime in Germany will, on past and present form, loose off another European war just so soon as it feels strong enough … we are considering very crude people, who have very few ideas in their noddles but brute force and militarism." Norman Rose, the author of Vansittart: Study of a Diplomat (1978) has argued: "But how would he combat the German menace? First, by redefining the aims of British strategy, by isolating Germany as Britain's most immediate danger, and then by boosting the British defence programme to meet this changed order of priorities. Well out of the public eye as a member of high-powered government committees, Vansittart laboured ceaselessly to realize these aims."

In 1934 Carr published a biography of Karl Marx. In the book, Karl Marx: A Study in Fanaticism, Carr attacked Marx's political ideas and claimed that it was motivated by "mindless class hatred". He also suggested that Marx's concept of the "dictatorship of the proletariat" was little different from the system being introduced in Nazi Germany. However, he warned that recent developments in the Soviet Union meant that Marx had "a claim to be regarded as the most far-seeing genius of the nineteenth century and one of the most successful prophets in history."

Carr favoured the appeasement policies of Stanley Baldwin and Neville Chamberlain. This upset Sir Robert Vansittart and played a role in Carr's resignation from the Foreign Office in 1936. Carr now became Professor of International Politics at the University of Wales. He continued to write extensively about politics and in 1937 was a strong supporter of non-intervention in the Spanish Civil War. Carr was also against the League of Nations having greater powers in order to stop international disputes: "I do not believe the time is ripe...for the establishment of a super-national force to maintain order in the international community and I believe any scheme by which nations should bind themselves to go to war with other nations for the preservation of peace is not only impracticable, but retrograde".

In 1937 Carr visited the Soviet Union and Nazi Germany. Carr was especially impressed by the achievements of Stalin's Five Year Plan. He argued in a speech at Chatham House later that year: "Both the German and Russian regimes, today, represent a reaction against the individualistic ideology prevailing at any, in Western Europe, for the last hundred and fifty years...The whole system of individualist laissez-faire economy has we know, broken down. It has broken down because production and trade can only be carried out on a nationwide scale and with the aid of State machinery and State control. Now, State control has come in its most naked and undisguised form precisely where the individualist tradition was the weakest, in Germany and Russia."

Carr published a defence of appeasement in his book, The Twenty Year Crisis (1939): "Having demolished the current utopia with the weapons of realism, we still need to build a new utopia of our own, which will one day fall to the same weapons. The human will will continue to seek escape from the logical consequences of realism in the vision of an international order which, as soon as it crystallizes itself into concrete political form, becomes tainted with self-interest and hypocrisy, and once more be attacked with the instruments of realism." The book was criticised by those on the left such as Norman Angell, who described it as a "completely mischievous piece of sophisticated moral nihilism" Arnold J. Toynbee wrote that after reading the book one was left "in a moral vacuum and at a political dead point".

Three months after the book was published the German Army invaded Poland. In March 1940, Carr was employed by the editor, Geoffrey Dawson, the editor of The Times as a leader writer. During the Second World War Carr's politics moved to the left. In one article he wrote: "The new order cannot be based on the preservation of privilege, whether the privilege be that of a country, of a class, or of an individual." In another editorial he argued: "This is not altogether a national war, it is to a certain extent a social war, a revolutionary war; as a political revolution it is not simply confined to one country but is more or less world-wide".

In 1942 Carr published Conditions of Peace. In the book Carr called for the creation of a socialist European federation. He argued that the war had been caused by a failure of capitalism and that the only way of preventing another world war was for the Western powers to fundamentally change the economic basis of their societies by adopting socialism. He also suggested that Britain should turn over all of her colonies to an international commission after the war. Carr later admitted: "Like a lot of other people, I took refuge in Utopian visions of a new world order after the war."

In The Soviet Impact on the Western World (1946) Carr argued that "The trend away from individualism and towards totalitarianism is everywhere unmistakable...The social and economic system of the Soviet Union, offering-as it does-almost unlimited possibilities of internal development, is hardly subject to those specific stimuli which dictated expansionist policies to capitalist Britain in the 19th century... there is nothing in Soviet policy so far to suggest that the east-west movement is likely to take the form of armed aggression or military conquest. The peaceful penetration of the Western world by ideas emanating from the Soviet Union has been, and seems likely to remain, a far important and conspicuous symptom of the new East-West movement."

After the war he associated with a group of left-wing historians that included Isaac Deutscher, Christopher Hill, Alan J. P. Taylor and Harold Laski. They were opposed to the Cold War and argued for better relations with the Soviet Union. Carr feared an outbreak of war: "It may be that the question whether war breaks out between Russia and America affects us far more than the question whether we can increase the productivity of labour or improve the organization of industry or the distribution of consumer goods. But the point is that we can hardly do anything about the first question and a great deal about the second". Carr insisted that "peace at any price must be the foundation of British policy".

Jonathan Haslam described Carr as: "Fervently individualist, ferociously intelligent, and scrupulously honest, Carr was by nature reserved and taciturn. Burdened by emotions with which he found it difficult to cope, he was always more relaxed in writing than in conversation. He none the less had a mordant wit and a certain charm when he chose to exercise it, delighting in gossip yet without ever entirely escaping a sense of guilt - doubtless stemming from a strict Victorian upbringing—at such reckless indulgence. No one ever claimed that he was boring."

In 1950 Carr embarked on a 14 volume A History of the Soviet Union. In 1953 Carr became tutor at Balliol College. Two years later he moved to Trinity College. His most popular work, What is History?, appeared in 1961. It included the controversial statement: "The facts are really not at all like fish on the fishmonger's slab. They are like fish swimming about in a vast and sometimes inaccessible ocean; and what the historian catches will depend, partly on chance, but mainly on what part of the ocean he chooses to fish in and what tackle he chooses to use – these two factors being, of course, determined by the kind of fish he wants to catch. By and large, the historian will get the kind of facts he wants. History means interpretation."

Edward Hallett Carr died of cancer on 5th November, 1982.