Sir Alexander Montagu George Cadogan OM GCMG KCB PC (25 November 1884 – 9 July 1968) was a British diplomat and civil servant.

His long tenure of the Permanent Secretary's office makes him one of the central figures of British policy before and during the Second World War.

Like most senior officials at the Foreign Office, he was bitterly critical of the appeasement policies of the 1930s but admitted that until British rearmament was better advanced, there were few other options.

His first posting was to Constantinople, where he "spent two happy years learning the craft of diplomacy and playing upon the head of Chancery a series of ingenious practical jokes.

"[7] Eden returned the admiration, writing that Cadogan "carried out his thankless task with a rare blend of intelligence, sensibility, and patience.

"[7] In 1933, with Adolf Hitler in power and the fate of the Disarmament Conference clear, Cadogan accepted a posting at the British legation in Peking.

[9] In 1936, Cadogan received a request from the newly appointed Secretary of State, Anthony Eden, offering him the post of joint Deputy Under-Secretary.

Assessing the situation, Cadogan advised a revision of the more vindictive elements of the Treaty of Versailles, "which was really more in the nature of an armistice.

Cadogan disagreed and wrote in his diary: "I believe that, so long as she is allowed to nurse her resentment to her bosom, her claims increase with her armaments.

Cadogan served in this capacity from 1938 to 1946 and represented Britain at the Dumbarton Oaks Conference in 1944, where he became well acquainted with Edward Stettinius and Andrei Gromyko.

Winston Churchill told Parliament, "His Majesty's Government could have had no abler representative that Sir Alexander Cadogan and there is no doubt that a most valuable task has been discharged.

"[12] Cadogan also accompanied Churchill to the Atlantic Conference in Placentia Bay, Newfoundland, aboard HMS Prince of Wales, acting as President Franklin D. Roosevelt in dress rehearsals to help him prepare.

[13] In preparation for the Yalta Conference, Cadogan expended a great deal of effort attempting to bring the "London Poles" under Stanislaw Mikolajczyk around to the idea of losing their eastern territories to the Soviet Union.

During his time at the UN, as David Dilks points out, "British diplomacy had to be conducted from a precarious position of over-commitment and economic instability.

"[18] Cadogan expressed great frustration with the inflexibility of his Soviet counterparts, who were forbidden from mixing with other delegations or informal exchanges of views.

[21] In 1952, Cadogan was made Chairman of Board of Governors of the BBC by Winston Churchill, who had returned to office the previous year.

[22] In the last decade of his life, Cadogan gradually shed his commitments and devoted more time to his late-blooming interest in art.