The Reserve Army was a field army

Army

An army An army An army (from Latin arma "arms, weapons" via Old French armée, "armed" (feminine), in the broadest sense, is the land-based military of a nation or state. It may also include other branches of the military such as the air force via means of aviation corps...

 of the British Army during World War I

British Army during World War I

The British Army during World War I fought the largest and most costly war in its long history. Unlike the French and German Armies, its units were made up exclusively of volunteers—as opposed to conscripts—at the beginning of the conflict...

 and part of the British Expeditionary Force during the First World War

World War I

World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...

. Under the command of Lieutenant-General Sir Hubert Gough

Hubert Gough

General Sir Hubert de la Poer Gough GCB, GCMG, KCVO was a senior officer in the British Army, who commanded the British Fifth Army from 1916 to 1918 during the First World War.-Family background:...

, the Reserve Army was formed on 23 May 1916 prior to the Battle of the Somme

Battle of the Somme (1916)

The Battle of the Somme , also known as the Somme Offensive, took place during the First World War between 1 July and 14 November 1916 in the Somme department of France, on both banks of the river of the same name...

 and was renamed the Fifth Army

British Fifth Army

The Fifth Army was a field army of the British Army during World War I and part of the British Expeditionary Force during the First World War.-History:...

 in October of that year.

The intended purpose of the army was to carry out the breakthrough phase of the Somme offensive once General Sir Henry Rawlinson's Fourth Army

British Fourth Army

The Fourth Army was a field army that formed part of the British Expeditionary Force during the First World War. The Fourth Army was formed on 5 February 1916 under the command of General Sir Henry Rawlinson to carry out the main British contribution to the Battle of the Somme.-History:The Fourth...

 had captured the German front-line trenches. For this role Gough was provided with the three British cavalry

Cavalry

Cavalry or horsemen were soldiers or warriors who fought mounted on horseback. Cavalry were historically the third oldest and the most mobile of the combat arms...

 divisions and in June he was allocated an infantry corps

Corps

A corps is either a large formation, or an administrative grouping of troops within an armed force with a common function such as Artillery or Signals representing an arm of service...

 of three divisions to support the advance. The army was nickname

Nickname

A nickname is "a usually familiar or humorous but sometimes pointed or cruel name given to a person or place, as a supposedly appropriate replacement for or addition to the proper name.", or a name similar in origin and pronunciation from the original name....

d "Gough's Mobile Army", a name which had first belonged to the 7th Division

British 7th Infantry Division

The 7th Infantry Division was established by Arthur Wellesley, 1st Duke of Wellington, as part of the Anglo-Portuguese Army, for service in the Peninsula War and was active also during the First World War from 1914-1918/19 and also in 1938-39 in Palestine and Egypt.-Peninsula War:The 7th Division...

 when Gough had been its commander. However on 21 June this infantry corps was withdrawn to GHQ reserve and the following day Gough's three cavalry divisions were placed under Fourth Army control, making the Reserve Army little more than a glorified corps.

In the evening of the first day on the Somme

First day on the Somme

The first day on the Somme, 1 July 1916, was the opening day of the Battle of Albert, which was the first phase of the British and French offensive that became known as the Battle of the Somme...

, 1 July 1916, the British Commander-in-Chief

Commander-in-Chief

A commander-in-chief is the commander of a nation's military forces or significant element of those forces. In the latter case, the force element may be defined as those forces within a particular region or those forces which are associated by function. As a practical term it refers to the military...

 General Sir Douglas Haig

Douglas Haig, 1st Earl Haig

Field Marshal Douglas Haig, 1st Earl Haig, KT, GCB, OM, GCVO, KCIE, ADC, was a British senior officer during World War I. He commanded the British Expeditionary Force from 1915 to the end of the War...

 relieved Rawlinson's Fourth Army of responsibility for the northern sector, placing the VIII and X Corps

X Corps (United Kingdom)

The X Corps was a British Army formation in the First World War and was later reformed in 1942 during the North African campaign of the Second World War as part of the Eighth Army.- First World War :...

 under Gough's command. Control of the cavalry divisions was retained by the Fourth Army. Gough would not officially assume his new command until 7am on 2 July but he immediately cancelled the orders for VIII Corps to resume the failed attack on Beaumont Hamel, thereby no doubt saving many lives.

The Reserve Army officially took responsibility for the northern sector on 4 July. Later II Corps, I Anzac Corps

I Anzac Corps

The I ANZAC Corps was a combined Australian and New Zealand army corps that served during World War I.It was formed in Egypt in February 1916 as part of the reorganisation and expansion of the Australian Imperial Force and the New Zealand Expeditionary Force following the evacuation of Gallipoli...

 and the Canadian Corps

Canadian Corps

The Canadian Corps was a World War I corps formed from the Canadian Expeditionary Force in September 1915 after the arrival of the 2nd Canadian Division in France. The corps was expanded by the addition of the 3rd Canadian Division in December 1915 and the 4th Canadian Division in August 1916...

 were also added to the Reserve Army strength.

For the next few months there were no major offensives on the Reserve Army front. The most significant effort was made at Pozières

Battle of Pozières

The Battle of Pozières was a two week struggle for the French village of Pozières and the ridge on which it stands, during the middle stages of the 1916 Battle of the Somme. Though British divisions were involved in most phases of the fighting, Pozières is primarily remembered as an Australian battle...

 and Mouquet Farm in late July and August.

The Reserve Army's first major battle was the Battle of Thiepval Ridge

Battle of Thiepval Ridge

The Battle of Thiepval Ridge was the first large offensive mounted by the British Reserve Army of Lieutenant General Hubert Gough during the Battle of the Somme and was designed to benefit from British Fourth Army's Battle of Morval by starting 24 hours afterwards...

 which began on 26 September. For much of October the Reserve Army carried out a series of attacks known as the Battle of the Ancre Heights

Battle of the Ancre Heights

The Battle of the Ancre Heights was a prolonged battle of attrition in October 1916 during the Battle of the Somme. Lieutenant General Hubert Gough's Reserve Army had finally managed to break out of the positions it had occupied since the start of the Somme fighting and Gough intended to maintain...

 and on 30 October the Reserve Army was renamed the Fifth Army

British Fifth Army

The Fifth Army was a field army of the British Army during World War I and part of the British Expeditionary Force during the First World War.-History:...

.