Cecil was born at Cavendish Square, London, the sixth child and third son of Robert Gascoyne-Cecil, 3rd Marquess of Salisbury, three times prime minister, and Georgina, daughter of Sir Edward Hall Alderson.

Cecil was a convinced believer in free trade, opposing Joseph Chamberlain's agitation for Tariff Reform, denouncing it as "a rather sordid attempt to ally Imperialism with State assistance for the rich".

[8] In March 1910 Cecil and his brother Lord Hugh, unsuccessfully appealed to Chamberlain that he should postpone advocating food taxes at the next election in order to concentrate on opposing Irish Home Rule.

Following the formation of the 1915 coalition government, he became Parliamentary Under-Secretary of State for Foreign Affairs on 30 May 1915; on 16 June he was sworn of the Privy Council,[11] and was promoted to Assistant Secretary in 1918–19.

He urged an alternative to war as a means of settling international disputes and claimed that neither the destruction of German militarism nor a postwar settlement based on self-determination would guarantee peace.

Cecil viewed the three months' delay before countries resorted to war as the principal role of the League as that would give public opinion time to exert its peaceful influence.

On 27 January Cecil and American legal expert David Hunter Miller spent four hours revising Wilson's proposals in what became known as the Cecil-Miller draft.

The primary solution was the construction of a European order on the basis of Christian morality, with a machinery of legal conciliation by which "Junkerism and Chauvinism" would be destroyed.

[42] A wider meeting (Cecil, Asquith, Grey and leading Asquithian Liberals Lord Crewe, Runciman and Sir Donald Maclean) was held on 5 July 1921.

With the fall of the Lloyd George coalition in October and the appointment of Bonar Law as Conservative prime minister, Cecil pledged to support the new government though he was not offered office.

[53] The Conservatives returned to power at the October 1924 general election and Cecil was asked by Stanley Baldwin to be Chancellor of the Duchy of Lancaster.

During a naval conference of 1927 in Geneva, negotiations broke down after the United States refused to agree that Britain needed a minimum of seventy cruisers for adequate defence of the British Empire, its trade and communications.

He was joint founder and president, with Pierre Cot, a French jurist, of the International Peace Campaign, known in France as Rassemblement universel pour la paix.

Baldwin told Thomas Jones on 27 February, "The very people like Bob Cecil who have made us disarm, and quite right too, are now urging us forward to take action.

[57] Cecil wrote to Baldwin in July that he found himself "more and more out of sympathy with modern Conservatism" and he considered the government's disarmament proposals made at Geneva "quite inadequate".

[58] In March 1933, he complained to Baldwin that the technical advisers, especially British ones, had sabotaged the prospect of abolishing aircraft and of bombing, particularly from those who wanted to retain it for areas such as the North-West frontier of India.

[66] In August he wrote to Murray that because Baldwin had quoted the "arch-militarist F. S. Oliver" in declaring that Britain's real frontier was on the Rhine, he was very far from a League frame of mind and that the government "ought to go" in spite of "the intellectual nonentity of the Labour party".

[66] He denounced the worldwide spread of nationalism and the outbreak of isolationism in Britain, claiming that isolation was a "principle of anarchy" and that in modern conditions countries could "no more live alone than individuals".

[67] The Stresa Front of 1935 between Britain, France and Italy received Cecil's criticism because it appeared to be an alliance in which Germany was excluded and condoned their failure to disarm.

But after its likely failure, the League would have reason for contemplating the "economic and financial measures which might be applied to a state endangering peace by the unilateral repudiation of its international obligations".

[74] In May 1938 Cecil complained that the government had "allowed the League to disintegrate" and in August that their "ambiguities and timidities" were failing to ensure that Hitler understood that further aggression would be a breach of international relations.

[75] In his memoirs, Cecil wrote that the wife of the Czechoslovak President, Edvard Beneš, telephoned him on behalf of her husband and asked for advice on the crisis: "I felt forced to reply that, much as I sympathized with her country, I could not advise her to rely on any help from mine.

[80] After the German invasion of the remaining Czechoslovak state in March 1939, Cecil was opposed to Eden rejoining the government because such a strengthening of Chamberlain would be a disaster.

[81] In his 1941 book A Great Experiment, Cecil strongly criticised Sir John Simon, the Foreign Secretary between 1931-1935 for his weak response to the Japanese seizure of Manchuria region of China in 1931, which he believed had led directly to World War Two.

"[83] He lived for thirteen more years, occasionally occupying his place in the House of Lords, and supporting international efforts for peace through his honorary life presidency of the United Nations Association.

He said that that system had been attacked by Russian dialectical materialism, "its central tenet is that there is no such thing as the spiritual nature of man, or, if there is, it should be ignored or stamped out as speedily as possible".

He was presented with honorary degrees by the Universities of Edinburgh, Oxford, Cambridge, Manchester, Liverpool, St Andrews, Aberdeen, Princeton, Columbia, and Athens.

I, myself, because my father was very keen and with him did much in the League of Nations field, remember Lord Robert Cecil coming to stay at home; and many a time at dinner, when I was a comparatively young man, I would watch him, with his long figure, slide more and more under the table, until only the distinguished head was left above his plate, and he would tell us of all his plans for the future peace of the world.

[87] Lord Pethick-Lawrence said of Cecil that his "life was devoted not to self, not to his own aggrandisement or some advantage of a personal kind, but to the well-being of his fellow human beings and the good fortune of this country and the whole world".

That cross hanging from his waistcoat pocket witnessed to the religious basis of his political faiths; but the sharp tongue, the determined chin, the large, powerful hand, the air of a man used to be obeyed, proud towards men if humble before God, did suggest that in that tall figure striding with his long legs the thronged corridors of the League, the levels of Christian charity were kept high above the plane of fools.