Belgium and France War broke out in the summer of 1914 and Horne, after departing for service in France with the first wave of the
British Expeditionary Force (BEF), "established a reputation as a skilful and experienced commander of artillery" whilst serving as BGRA to Lieutenant General Sir Douglas Haig, who commanded
I Corps. At the
Battle of Mons, Horne "was given command of the Corps rearguard by Haig and performed with great success". Horne fought with distinction in the BEF's actions throughout 1914; in October of that year, he was promoted to
major general, “for distinguished conduct in the Field”. A few months later, in January 1915, he was given command of the
2nd Division. In May, Horne's division participated in the first British night attack of the war, distinguishing itself at the
Battle of Festubert; the attack faltered, partly because the artillery ran out of ammunition. The media launched vicious attacks on the
Secretary of State for War, Field Marshal
Lord Kitchener; the blame was eventually laid on Field Marshal French who was forced to resign at the year's end. Significantly, the artillery were reorganised after this fiasco at Horne's suggestion. Under his command, the division also saw significant action during the
Battle of Loos.
Middle East In November 1915, Horne accompanied Lord Kitchener to the
Dardanelles, where they organised and executed the
evacuation of Gallipoli which took place in December 1915 and January 1916. For several months after this, Horne was placed in charge of the
Suez Canal defences (and given command of the
XV Corps). and still GOC XV Corps, he returned to the
Western Front. His corps was allotted to General
Sir Henry Rawlinson's
Fourth Army, which was preparing for an attack in the
Somme area. In the pre-battle plans, Horne advocated and became an architect of the "creeping
barrage", a tactic that was used for the rest of the war. On 1 July 1916, the
first day on the Somme, Horne's XV Corps participated in the costliest British battle of the First World War. His force, consisting of the
7th and
21st divisions, attacked the villages of
Fricourt and
Mametz, capturing both on the first day, although suffering 7,500 casualties in the process. , GOC
Canadian Corps, at the First Army Commemoration Service of the beginning of the fourth year of the First World War,
Ranchicourt, France, 5 August 1917. French officers are also present. The divisions bypassed Mametz Wood, a position the Germans had heavily entrenched and needed to be captured to allow Horne's XV Corps to carry on the advance. As the 7th Division had suffered heavy casualties, the
38th (Welsh) Division was assigned to the corps and ordered to take the wood. Horne interfered in the division's efforts to attack the wood, issuing conflicting orders and going as far as to ordering a single platoon into action. Due to the miscommunication between Horne and the division's commander, Major General
Ivor Philipps, was fired and replaced by the commander of the 7th Division, Major General
Herbert Watts. Horne wrote a "self-serving" account of this event that did no justice to men of the division or the difficulties they had faced. On 9 July the Welsh launched a full-scale attack on the woods and cleared it by the following day. During their 6 days on the Somme, the Welsh division suffered 3,993 casualties. Historian Don Farr wrote that the reputation of the Welsh division suffered due to the repeated interference by Horne in matters best left to the divisional or brigade staff and his "inexperience of battlefield command at this level". With the wood cleared, Horne would lead his corps during the
Bazentin Ridge, the
Battle of Delville Wood, and the
Battle of Flers–Courcelette. and General Horne inspecting men of the 2/6th Battalion,
South Staffordshire Regiment,
59th (2nd North Midland) Division, at Gauchin, 30 March 1918. They are accompanied by Brigadier General
T. G. Cope and Major General
Cecil Romer, GOC 59th Division.
Army commander In September 1916, Horne was created a
Knight Commander of the Order of the Bath. After the successful capture of
Flers, he was promoted to temporary
general and succeeded General
Sir Charles Monro (after a brief hiatus between 7 August and 29 September when the command was held by Haig's first choice, Lieutenant General
Richard Haking, who was then blocked from receiving the promotion) as commander of the First Army. His first trial occurred in April 1917, when his troops were sent on a diversionary attack on the fearsome
Vimy Ridge, which rose hundreds of feet over the surrounding landscape.
French Army commander
Robert Nivelle was critical of Horne's plan; Nivelle was the one found incompetent and, after one month of relative failure (and the beginnings of the
1917 French Army mutinies), Nivelle was sacked and replaced with
Philippe Pétain. The attack on Vimy Ridge was spearheaded by the First Army's "shock troops" (the
Canadian Corps, under Lieutenant Lieutenant General
The Hon. Sir Julian Byng, later replaced by the Canadian Lieutenant General
Arthur Currie). The ensuing
Battle of Vimy Ridge, the first of a series of actions known as the
Battle of Arras was successful: supported by Horne's 1,000-odd artillery pieces, the Canadian forces took the ridge in four days, with approximately 10,000 casualties (against 20,000 German casualties). The capture of Vimy Ridge would prove essential to the British Army: it served as the backbone of the British defence from March 1918 onwards. , at Dieval, 12 June 1918. The motorbikes are Clyno 744 cc twin cylinder machines fitted with a sidecar and Vickers machine-guns. Nivelle's failure and sacking lengthened the actions around Arras. With success imminent, Haig began siphoning troops northward, where many would participate in the
Battles of Messines and
Passchendaele. The First Army served mainly as a diversion and a placeholder until April 1918. (centre front) with his senior commanders and staff officers at
Cambrai, France, 11 November 1918. Stood in the second row, first on the right, is General Sir Henry Horne. In April, the Germans embarked on the
Spring Offensive which was similar to the Allied Somme Offensive two years previously. At first, the attack was successful. On Horne's front, nine German divisions attacked his weak left flank which was manned by two exhausted
Portuguese divisions. The Germans advanced six miles to the banks of the
River Lawe, where they were repulsed by the
55th (West Lancashire) and the
51st (Highland) divisions. After this final German offensive, the British took the initiative permanently. Haig's forces embarked on the
Hundred Days Offensive, which ended the war; Horne's troops distinguished themselves in the lengthy offensive. On the last day of the war Horne wrote to his wife: ==Post-war==