As Sub Chief of Staff to the British Expeditionary Force (BEF), Wilson was Sir John French's most important advisor during the 1914 campaign, but his poor relations with Haig and Robertson saw him sidelined from top decision-making in the middle years of the war.

Wilson's brigade took part in the Battle of Colenso (15 December), in which British troops, advancing after an inadequate artillery bombardment, were shot down by entrenched and largely hidden Boers armed with magazine rifles.

[46] Wilson gained both the substantive promotion to major and the promised brevet in December 1901,[47] and in 1902 became Commanding Officer of the 9th Provisional Battalion, Rifle Brigade at Colchester,[23][48] intended to supply drafts for the South African War, then still in progress.

Field Marshal Roberts wrote to Richard Haldane (Secretary of State for War) and Esher recommending Wilson on the basis of his excellent staff work in South Africa, and as a strong character needed to maintain Rawlinson's improvements at Camberley.

[68] In speeches to students, Wilson stressed the need for administrative knowledge ("the drudgery of staffwork"), physical fitness, imagination, "sound judgement of men & affairs" and "constant reading & reflexion on the campaigns of the great masters".

[68] In April and May 1910, with his term of office at Camberley still officially running until January 1911, the Chief of the Imperial General Staff (CIGS), William Nicholson, told Wilson that he was to succeed Spencer Ewart as Director of Military Operations that summer and vetoed him from accepting Horace Smith-Dorrien's offer of a brigade at Aldershot.

The Wilson-Dubail memorandum, although making explicit that neither government was committed to action, promised that in the event of war the Royal Navy would transport 150,000 men to Rouen, Le Havre and Boulogne, and that the BEF would concentrate between Arras, Cambrai and St Quentin by the thirteenth day of mobilisation.

Jemmy had been on the platform in Belfast in April 1912 when Law addressed a mass meeting against Home Rule, and in the summer of 1912 he came to London to work for the Ulster Defence League (run by Walter Long and Charlie Hunter).

[116] In September 1912 he inspected Warsaw with Alfred Knox, British military attaché in Russia, then met Zhilinsky in St Petersburg, before visiting the battlefield of Borodino, and Kiev, then – in Austria-Hungary – Lemburg, Krakow and Vienna.

[120] Wilson's support for conscription made him friendly with Leo Amery, Arthur Lee, Charlie Hunter, Earl Percy, (Lord) Simon Lovat, Garvin of The Observer, Gwynne of The Morning Post and F.S Oliver, owner of the Department Store Debenham and Freebody.

His mission was not secret – the official purpose was to inspect 3rd Royal Irish Rifles and give a lecture on the Balkans at Victoria Barracks, and he reported his opinion of the Ulster situation to the Secretary of State and to Sir John French – but attracted press speculation.

[172] Baker-Carr recalled Wilson's standing in dressing gown and slippers uttering "sardonic little jests to all and sundry within earshot" as GHQ packed up to evacuate, behaviour which historian Dan Todman comments was probably "reassuring for some but profoundly irritating for others".

[189] Leo Maxse, H. A. Gwynne and the radical Josiah Wedgwood MP, impressed by Wilson's support for conscription[197] and for the abandonment of Gallipoli, tipped him as a potential CIGS in place of James Wolfe-Murray, but Archibald Murray was appointed instead (September 1915).

[201][202][203] Wilson attended the Anglo-French Chantilly Conference (6–8 December 1915) along with Murray (CIGS), French and Robertson, as well as Joffre, Maurice Pellé and Victor Huguet for France, Zhilinski and Ignatieff for Russia, Cadorna for Italy and a Serb and Belgian representative.

[217] Wilson thought that "to slog on at one spot" on the Somme was "dreadfully lacking in imagination" and would have preferred a joint offensive by Russia, Italy and Romania in Spring 1917 to draw off 15 or 20 German divisions, allowing the BEF to "completely smash the Boch line".

Éamon de Valera of Sinn Féin had recently won the East Clare by-election and on a visit to Currygrane (his first in eight years) everyone Wilson spoke to – judges, landowners, police officers, a Redmondite local politician and "some natives" agreed on the need for Irish conscription.

[229] Brock Millman argues that the threat to stand for Parliament, where he could have intrigued as a Unionist along with his friends such as Bonar Law, Leo Amery and Colonel Lord Percy, ADC to the King, was blackmail to get a military job out of Lloyd George.

Dining with Wilson and French the night before, Lloyd George criticised Robertson and called Haig's recent paper (8 October), which predicted that "decisive success is expected next year" provided Russia continued to pin down as many German divisions as currently, "preposterous".

Winston Churchill later wrote "In Sir Henry Wilson the War Cabinet found for the first time an expert advisor of superior intellect, who could explain lucidly and forcefully the whole situation and give reasons for the adoption or rejection of any course".

[245] The military representatives, egged on by Wilson, beginning 13 December 1917, recommended coordinated defence and reserves from the North Sea to the Adriatic, as well as reorganisation of the Belgian Army and preparing studies of the Italian and Salonika fronts.

There was talk of the government falling, Rawlinson writing to H. A. Gwynne (14 February 1918) that the best solution was to give Robertson a powerful role at Versailles and have Wilson as a weak CIGS in London "where he will not be able to do much mischief – especially if Squiff replaced LG as PM".

There is no evidence to confirm Haig's later claim that, on returning from a midnight meeting with Petain at 3am on 25 March, he telegraphed to Wilson and Milner to come over to France and ensure the appointment of "Foch or some other determined general who would fight" as Allied Generalissimo.

He also wrote to Foch (10 April) urging him to send French reinforcements or to flood the coastal areas around Dunkirk, and impressing on him the need to keep contact with the British right flank if the BEF felt compelled to retreat on the Channel Ports.

[294] When Haig's forces began to advance towards the Hindenburg Line Wilson sent him a supposedly "personal" telegram (31 August), warning that he was not to take unnecessary losses in storming these fortifications (i.e. hinting that he might be sacked if he failed), later claiming that the government wanted to retain troops in the UK because of the police strike.

By October 1920 the local British commander Sir Aylmer Haldane managed to restore order but on 10 December Wilson minuted his agreement to an appraisal by the Director of Military Operations that "we ran things too fine and that a great disaster was only narrowly avoided".

[333] Wilson became increasingly concerned that Henry Hugh Tudor, with the connivance of Lloyd George, who loved to drop hints to that effect, was operating an unofficial policy of killing IRA men in reprisal for the deaths of pro-Crown forces.

Wilson wrote to Macready (June 1920) that "the discipline and good name of the Army is worth half a dozen Irelands" – although sympathetic, he had been deeply concerned to hear of troops smashing up Fermoy in reprisal for the kidnapping of General Cuthbert.

At a conference on St Patrick's Day 1922 Wilson advised an increase in the Special Constabulary, but urged that loyal Catholics be encouraged to join, rather than keeping it a purely Protestant body (Craig did not pass on this recommendation to the Stormont Cabinet).

[363][incomplete short citation] Tim Pat Coogan has suggested that Reginald Dunne, who had the confidence of both Michael Collins and Rory O'Connor, undertook the shooting as a last-ditch effort to provoke the British Government into retaliating, thereby uniting both sides of the Nationalists.

What a Lovely War (1969), travelling in a car in August 1914 with a cretinous Sir John French (Laurence Olivier) who rejects his offer to arrange an interpreter as it might breach the need for "absolute secrecy", but later being passed over in favour of Robertson for a staff promotion.