[2][5][6] According to historian Richard Doherty, Dill's conduct at Sandhurst were "exemplary" although there "were no indications of the outstanding intelligence for which he would later be noted and his marks were described as mediocre".

[9] After the end of the war in June 1902, Dill left Cape Town with other men of his battalion on the SS Englishman in late September 1902, arriving at Southampton the following month, from where they were posted to Fermoy.

[16][17] Staff College certainly made Dill; his talents for planning and problem solving were discovered and honed during his year at Camberley and were soon put to the toughest test of all – war.

[16][17] From the spring of 1918 he was Head of Operations at GHQ, an important promotion after the sacking of many of Field Marshal Sir Douglas Haig's senior staff following the Battle of Cambrai.

[40] At the outbreak of the Second World War Dill hoped to be appointed Commander-in-Chief of the British Expeditionary Force, but the position again went to Gort.

The resulting vacancy as Chief of the Imperial General Staff (CIGS) was filled by Sir Edmund Ironside, leaving Dill to be eventually posted as commander of I Corps in France on 3 September 1939.

[42][5] On returning to the UK in April 1940,[43] Dill was appointed Vice Chief of the Imperial General Staff (and a member of the Army Council),[44] under Ironside, by the then Prime Minister Neville Chamberlain.

Although acknowledging that first use of chemical weapons would alienate the United States and invite retaliation, he concluded that "at a time when our National existence is at stake ... we should not hesitate to adopt whatever means appear to offer the best chance of success.

He also served on the Combined Policy Committee set up by the British and United States governments under the Quebec Agreement to oversee the construction of the atomic bomb.

[53] In the United States he was immensely important in making the Chiefs of Staff committee – which included members from both countries – function, often promoting unity of action.

[54] He was particularly friendly with General George Marshall[55] and the two exercised a great deal of influence on President Roosevelt who described Dill as "the most important figure in the remarkable accord which has been developed in the combined operations of our two countries".

He was sorely missed by the American Joint Chiefs of Staff, who sent a warm message of condolence to their British colleagues: We feel we share equally with you the loss to our combined war effort resulting from the death of Field Marshal Sir John Dill.