The British Army is the principal land warfare force of the United Kingdom, a part of British Armed Forces. As of 2018, the British Army comprises just over 81,500 trained regular (full-time) personnel and just over 27,000 trained reserve (part-time) personnel.
The modern British Army traces back to 1707, with an antecedent in the English Army that was created during the Restoration in 1660. The term "British Army" was adopted in 1707 after the Acts of Union between England and Scotland. Although all members of the British Army are expected to swear (or affirm) allegiance to Elizabeth II as their commander-in-chief, the Bill of Rights of 1689 requires parliamentary consent for the Crown to maintain a peacetime standing army; hence the reason it is not called the "Royal Army". Therefore, Parliament approves the Army by passing an Armed Forces Act at least once every five years. The Army is administered by the Ministry of Defence and commanded by the Chief of the General Staff.
The British Army has seen action in major wars between the world's great powers, including the Seven Years' War, the Napoleonic Wars, the Crimean War and the First and Second World Wars. Britain's victories in these decisive wars allowed it to influence world events and establish itself as one of the world's leading military and economic powers. Since the end of the Cold War the British Army has been deployed to a number of conflict zones, often as part of an expeditionary force, a coalition force or part of a United Nations peacekeeping operation.
Until the English Civil War, England never had a standing army with professional officers and careerist corporals and sergeants. It relied on militia organized by local officials, or private forces mobilized by the nobility, or on hired mercenaries from Europe. From the later Middle Ages until the English Civil War, when a foreign expeditionary force was needed, such as the one that Henry V of England took to France and that fought at the Battle of Agincourt (1415), the army, a professional one, was raised for the duration of the expedition.
During the English Civil War, the members of the Long Parliament realised that the use of county militia organised into regional associations (such as the Eastern Association), often commanded by local members of parliament (both from the House of Commons and the House of Lords), while more than able to hold their own in the regions which Parliamentarians controlled, were unlikely to win the war. So Parliament initiated two actions. The Self-denying Ordinance, with the notable exception of Oliver Cromwell, forbade members of parliament from serving as officers in the Parliamentary armies. This created a distinction between the civilians in Parliament, who tended to be Presbyterian and conciliatory to the Royalists in nature, and a corps of professional officers, who tended to Independent politics, to whom they reported. The second action was legislation for the creation of a Parliamentary-funded army, commanded by Lord General Thomas Fairfax, which became known as the New Model Army (originally new-modelled Army).
While this proved to be a war winning formula, the New Model Army, being organized and politically active, went on to dominate the politics of the Interregnum and by 1660 was widely disliked. The New Model Army was paid off and disbanded at the Restoration of the monarchy in 1660. For many decades the excesses of the New Model Army under the Protectorate of Oliver Cromwell was a horror story and the Whig element recoiled from allowing a standing army. The militia acts of 1661 and 1662 prevented local authorities from calling up militia and oppressing their own local opponents. Calling up the militia was possible only if the king and local elites agreed to do so.
Charles II and his Cavalier supporters favoured a new army under royal control; and immediately after the Restoration began working on its establishment. The first English Army regiments, including elements of the disbanded New Model Army, were formed between November 1660 and January 1661 and became a standing military force for Britain (financed by Parliament). The Royal Scots and Irish Armies were financed by the parliaments of Scotland and Ireland. Parliamentary control was established by the Bill of Rights 1689 and Claim of Right Act 1689, although the monarch continued to influence aspects of army administration until at least the end of the nineteenth century.