The history of the British Army spans over three and a half centuries and numerous European wars, colonial wars and world wars. From the early 19th century until 1914, the United Kingdom was the greatest economic and imperial power in the world, and although this dominance was principally achieved through the strength of the British Royal Navy, the British Army played a significant role.

The British Army came into being with the merger of the Scottish Army and the English Army, following the unification of the Kingdom of England and the Kingdom of Scotland, as the Kingdom of Great Britain in 1707. The new British Army incorporated existing English and Scottish regiments, and was controlled from London.

From about 1763, Great Britain and its successor the United Kingdom has been one of the leading military and economic powers of the world.

In peacetime, Britain has generally maintained only a small professional volunteer army, expanding this as required in time of war, due to Britain's traditional role as a sea power. Since the suppression of Jacobitism in 1745, the army has played little role in British domestic politics (except for the Curragh mutiny), and, other than in Ireland, has seldom been deployed against internal threats to the state (one notorious exception being the Peterloo Massacre).

The Army has been involved in many global international conflicts, including the Napoleonic Wars, the Crimean War and the two World Wars. Historically, it contributed to the expansion and retention of the British Empire.

The British Army has long been at the forefront of new military developments. It was the first in the world to develop and deploy the tank, and what is now the Royal Air Force (RAF) had its origins within the British Army as the Royal Flying Corps (RFC). At the same time the Army emphasises the continuity and longevity of several of its institutions and military traditions.

Early British Empire

The British Empire expanded in this time to include colonies, protectorates, and Dominions throughout the Americas, Africa, Asia and Australasia. Although the Royal Navy is widely regarded as having been vital for the rise of the British Empire, and British dominance of the world, the British Army played an important role in the colonisation of India and other regions. Typical tasks included garrisoning the colonies, capturing strategically important territories, and participating in actions to pacify colonial borders, provide support to allied governments, suppress Britain's rivals, and protect against foreign powers and hostile natives.

British troops also helped capture strategically important territories, allowing their empire to expand throughout the globe. The army also involved itself in numerous wars meant to pacify the borders, or to prop-up friendly governments, and thereby keep other, competitive, empires away from the British Empire's borders. Among these actions were the Seven Years' War, the American Revolutionary War, the Napoleonic Wars, the First and Second Opium Wars, the Boxer Rebellion, the New Zealand land wars, the Sepoy Rebellion of 1857, the First and Second Boer Wars, the Fenian raids, the Irish War of Independence, its serial interventions into Afghanistan (which were meant to maintain a friendly buffer state between British India and the Russian Empire), and the Crimean War (to keep the Russian Empire at a safe distance by coming to Turkey's aid).

As had its predecessor, the English Army, the British Army fought Spain, France, and the Netherlands for supremacy in North America and the West Indies. With native and provincial assistance, the Army conquered New France in the Seven Years' War and subsequently suppressed a Native American uprising in Pontiac's War. The British Army suffered defeat in the American War of Independence, losing the Thirteen Colonies but holding on to Canada.

The British Army was heavily involved in the Napoleonic Wars in which the army served in multiple campaigns across Europe (including continuous deployment in the Peninsular War), the Caribbean, North Africa and later in North America. The war between the British and the First French Empire of Napoleon Bonaparte stretched around the world and at its peak, in 1813, the regular army contained over 250,000 men. A British Army under the Duke of Wellington defeated Napoleons last campaign at the Battle of Waterloo in 1815.

The English had been involved, both politically and militarily, in Ireland since being given the Lordship of Ireland by the pope in 1171. English republican dictator, Oliver Cromwell's campaign was characterised by its uncompromising treatment of the Irish towns (most notably Drogheda) that had supported the Royalists during the English Civil War. The English Army (and subsequently the British Army)stayed in Ireland primarily to suppress numerous Irish revolts and campaigns for independence. It was faced with the prospect of battling Anglo-Irish and Ulster Scots peoples in Ireland, who alongside their other Irish groups had raised their own volunteer army and threatened to emulate the American colonists if their conditions were not met. The British Army found itself fighting Irish rebels, both Protestant and Catholic, primarily in Ulster and Leinster (Wolfe Tone's United Irishmen) in the 1798 rebellion.

In addition to battling the armies of other European Empires' (and of its former colonies, the United States, in the American War of 1812), in the battle for global supremacy, the British Army fought the Chinese in the First and Second Opium Wars, and the Boxer Rebellion, Māori tribes in the first of the New Zealand Wars, Nawab Shiraj-ud-Daula's forces and British East India Company mutineers in the Sepoy Rebellion of 1857, the Boers in the First and Second Boer Wars, Irish Fenians in Canada during the Fenian raids and Irish separatists in the Anglo-Irish War.

Following William and Mary's accession to the throne, England involved itself in the War of the Grand Alliance primarily to prevent a French invasion restoring Mary's father, James II. Following the 1707 union of England and Scotland, and the 1801 creation of the United Kingdom of Great Britain and Ireland, British foreign policy, on the continent, was to contain expansion by its competitor powers such as France and Spain. The territorial ambitions of the French led to the War of the Spanish Succession and the Napoleonic Wars. Russian activity led to the Crimean War.

The vastly increasing demands of imperial expansion, and the inadequacies and inefficiencies of the underfunded, post-Napoleonic Wars British Army, and of the Militia, Yeomanry, and Volunteer Force, led to the Cardwell and Childers Reforms of the late 19th century, which gave the British Army its modern shape, and redefined its regimental system. The Haldane Reforms of 1907, formally created the Territorial Force as the Army's volunteer reserve component.

World Wars

Great Britain's dominance of the world had been challenged by numerous other powers, notably Germany. The UK was allied with France (by the Entente Cordiale) and Russia, and when the First World War broke out in 1914, the British Army sent the British Expeditionary Force to France and Belgium to prevent Germany from occupying these countries. The War would be the most devastating in British military history, with near 800,000 men killed and over 2 million wounded. In the early part of the war, the professional force of the BEF was decimated and, by turns, a volunteer (and then conscripted) force replaced it. Major battles included the Battle of the Somme. Advances in technology saw advent of the tank, with the creation of the Royal Tank Regiment, and advances in aircraft design, with the creation of the Royal Flying Corps, which were to be decisive in future battles. Trench warfare dominated strategy on the Western Front, and the use of chemical and poison gases added to the devastation.

The Second World War broke out in 1939 with the German invasion of Poland. British assurances to the Polish led the British Empire to declare war on Germany. Again an Expeditionary Force was sent to France, only to be hastily evacuated as the German forces swept through the Low Countries and across France in 1940. Only the Dunkirk evacuation saved the entire Expeditionary Force from capture. Later, however, the British would have spectacular success defeating the Italians and Germans at the Battle of El Alamein in North Africa, and in the D-Day invasion of Normandy with the help of American, Canadian, Australian and New Zealand forces. Contrary to popular belief, over half of the Allied soldiers on D-day were British. In the Far East, the British army battled the Japanese in Burma. The Second World War saw the British army develop its Special Air Service, Commando units and the Parachute Regiment. During the war the British army was one of the major fighting forces on the allied side.

Postcolonial era

After the end of the Second World War, the British Army was significantly reduced in size, although National Service continued until 1960. This period also saw the process of Decolonisation commence with the end of the British Raj, and the independence of other colonies in Africa and Asia. Accordingly the army's strength was further reduced, in recognition of Britain's reduced role in world affairs, outlined in the 1957 Defence White Paper. This was despite major actions in Korea in 1950 and Suez in 1956. A large force of British troops also remained in Germany, facing the threat of Soviet invasion. The British Army of the Rhine was the Germany garrison formation, with the main fighting force being I (BR) Corps. The Cold War saw significant technological advances in warfare and the Army saw more technologically advanced weapons systems come into service.

Despite the decline of the British Empire, the Army was still deployed around the world, fighting colonial wars in Aden, Cyprus, Kenya and Malaya. In 1982 the British Army, alongside the Royal Marines, helped to recapture the Falkland Islands during the war against Argentina.

In the three decades following 1969, the Army was heavily deployed in Northern Ireland, to support the Royal Ulster Constabulary (later the Police Service of Northern Ireland) in their conflict with republican paramilitary groups, called Operation Banner. The locally-recruited Ulster Defence Regiment was formed, later becoming the Royal Irish Regiment in 1992. Over 700 soldiers were killed during the Troubles. Following the IRA ceasefires between 1994 and 1996 and since 1997, demilitarisation has taken place as part of the peace process, reducing the military presence from 30,000 to 5,000 troops. On 25 June 2007, the Second Battalion Princess of Wales's Royal Regiment vacated the Army complex at Bessbrook Mill in Armagh. This is part of the 'normalisation' programme in Northern Ireland in response to the IRA's declared end to its activities.