Northern Ireland is one of the four countries of the United Kingdom (although it is also described by official sources as a province or a region), situated in the north-east of the island of Ireland. It was created as a separate legal entity on 3 May 1921 under the Government of Ireland Act 1920.Statutory Rules & Orders published by authority, 1921 (No. 533); Additional source for 3 May 1921 date: The new autonomous Northern Ireland was formed from six of the nine counties of Ulster: four counties with unionist majorities Antrim, Armagh, Down, and Londonderry and two counties with slight Irish nationalist majorities Fermanagh and Tyrone in the 1918 general election. The remaining three Ulster counties with larger nationalist majorities were not included. In large part unionists, at least in the north-east, supported its creation while nationalists were opposed.

The Troubles in Northern Ireland (1920–1922) were followed by decades of relatively peaceful rule by the Ulster Unionist Party-controlled government of Northern Ireland (1921–1972), interrupted by Luftwaffe attacks during World War II. Systemic discrimination against Catholics by the government ensured a high emigration rate from that community and contributed to its continued dislike of the partition of Ireland. The Troubles erupted in the late 1960s, and continued until the 1998 Belfast Agreement.

Resistance to Home Rule From the late 19th century, the majority of people living in Ireland wanted the British government to grant some form of self-rule to Ireland. The Irish Parliamentary Party (IPP) sometimes held the balance of power in the House of Commons in the late 19th and early 20th centuries, a position from which it sought to gain Home Rule, which would have given Ireland autonomy in internal affairs, without breaking up the United Kingdom. Two bills granting Home Rule to Ireland were passed by the House of Commons in 1886 and 1893, but rejected by the House of Lords. With the passing of the Parliament Act 1911 by the Liberal Party government (which reduced the powers of the Lords from striking down parliamentary Bills to delaying their implementation for two years), it was apparent that Home Rule would probably come into force in the next five years. The IPP and its predecessor, the Home Rule League, had been campaigning for this for almost fifty years.

However, a significant minority was vehemently opposed to the idea and wished to retain the Union in its existing form. Irish unionists had been agitating successfully against Home Rule since the 1880s, and on 28 September 1912, their leader, Edward Carson, introduced the Ulster Covenant in Belfast, pledging to exclude Ulster from home rule. The Covenant was signed by 450,000 men. Whilst precipitating a split with unionists in the south and west (including a particularly sizeable community in Dublin), it gave northern unionists a feasible goal to aim for.

By the early 20th century, Belfast, the largest city in Ulster, had become the largest city in Ireland. Its industrial economy, with strong engineering and shipbuilding sectors, was closely integrated with that of Great Britain. Belfast was a substantially Ulster Protestant city with a Catholic minority of less than 30 per cent, concentrated in the west of the city.

A third Home Rule Bill was introduced by the Liberal minority government in 1912. However, the Conservative Party was sympathetic to the unionist case, and so the political voice of Irish unionism was amplified in Parliament. After heavy amendment by the House of Lords, the Commons agreed in 1914 to allow four counties of Ulster to vote themselves out of its provisions for six years. Throughout 1913 and 1914, paramilitary militias were recruited and armed, firstly the unionist Ulster Volunteer Force (UVF), and in response, the nationalist Irish Volunteers. In reaction to the possibility of the UVF seizing weapons, units of the British army stationed in Ireland were ordered to occupy government buildings, to repel any assaults by the UVF and to guard armories. When (March 1914) orders were issued transferring officers from the main army base at the Curragh to the north of Ireland, numerous officers chose to resign their commissions rather than obey.Bromage, Mary (1964), Churchill and Ireland, University of Notre Dame Press, Notre Dame, IL, pg 44, Library of Congress Catalog Card Number 64-20844 See: Curragh incident. Events in Europe were to take precedence after the outbreak of World War I. Home rule was delayed for the duration of what was expected to be a short war, and unionist and nationalist leaders agreed to encourage their volunteers to join the British army.

1916: Easter Rising, Battle of the Somme and aftermath During World War I, tensions continued to mount in Ireland. Hardline Irish separatists (known at the time as Irish Nationalists and later as Republicans) rejected Home Rule entirely because it involved maintaining the connection with Britain. They retained control of one faction of the Irish Volunteers, and in Easter 1916, led by Thomas Clarke, James Connolly and others attempted a rebellion in Dublin. After summary trials, the British government had the leaders executed for treason. The government blamed the small Sinn Féin party, which had little to do with it. The execution of the leaders of the rebellion turned out to be a propaganda coup for militant republicanism, and Sinn Féin's previously negligible popular support grew. The surviving leaders of the Irish Volunteers infiltrated the party and assumed its leadership in 1917. (The Irish Volunteers would later become the Irish Republican Army (IRA) in 1919.) Republicans gained further support when the British government attempted to introduce conscription to Ireland in 1918. Sinn Féin was at the forefront of organising the campaign against conscription.

The 36th (Ulster) Division was one of the first units in the British Army to be sent into the Somme beginning in July 1916. Despite being one of the few divisions to achieve their objectives, the Ulstermen suffered nearly 85% casualties. Though the 36th Division was made up of both Catholics and Protestants from the north, one result from the heavy losses at the Somme was that the Unionist community became evermore determined to remain in the United Kingdom, believing themselves to have sacrificed their sons at the behest of the Crown. Nationalists joined in great numbers as well, with "old" Irish regiments from Munster and Leinster being greatly strengthened by these recruits. When the veterans of World War I, on both sides of the political divide, returned from the Front in 1918 and 1919, they came back as battle-hardened soldiers. In the 1918 general election, the Irish Parliamentary Party lost almost all of its seats to Sinn Féin. Of the 30 seats in the six counties that would become Northern Ireland, 23 were won by Unionists, including three Labour Unionists and five of the six IPP members returned in Ireland were elected in Ulster as a result of local voting pacts with Sinn Féin.

Guerrilla warfare gathered pace in Ireland in the aftermath of the election, leading to the Anglo-Irish War. Although lower in intensity in Ulster than the rest of Ireland, the conflict was complicated there by involving not only the IRA, British Army and Royal Irish Constabulary, but the Ulster Volunteer Force (UVF) as well.

Partition

The fourth and final Home Rule Bill (the Government of Ireland Act 1920) partitioned the island into Northern Ireland (six northeastern counties) and Southern Ireland (the rest of the island). Some unionists such as Sir Edward Carson opposed partition, seeing it as a betrayal of unionism as a pan-Irish political movement. Three Counties unionists (those living in the Ulster counties of Cavan, Donegal, and Monaghan) who found themselves on the wrong side of the new border that partitioned Ulster, felt betrayed by those who had joined them in pledging to "stand by one another" in the Ulster Covenant. The Belfast Telegraph reassured unionists who felt guilty about this "that it was better for two-thirds of passengers to save themselves than for all to drown". Many Irish nationalists also opposed partition, although some were gratified that Northern Ireland contained a large nationalist minority that would deny it stability.

The Treaty was given effect in the United Kingdom through the Irish Free State Constitution Act 1922. Under Article 12 of the Treaty, Northern Ireland could exercise its opt out by presenting an address to the King requesting not to be part of the Irish Free State. Once the Treaty was ratified, the Parliament of Northern Ireland had one month to exercise this opt out during which month the Irish Free State Government could not legislate for Northern Ireland, holding the Free State's effective jurisdiction in abeyance for a month.

On 7 December 1922 (the day after the establishment of the Irish Free State) the Parliament of Northern Ireland resolved to make the following address to the King so as to opt out of the Irish Free State:

On 13 December 1922 Prime Minister James Craig addressed the Parliament of Northern Ireland informing them that the King had responded to the Parliament's address as follows:

Early years of Home Rule Northern Ireland, having received self-government within the United Kingdom under the Government of Ireland Act, was in some respects left to its own devices.

The first years of the new autonomous region were marked by bitter violence, particularly in Belfast. The IRA was determined to oppose the partition of Ireland so the authorities created the (mainly ex-UVF) Ulster Special Constabulary to aid the Royal Irish Constabulary (RIC) and introduced emergency powers to combat the IRA. Many died in political violence between 1920 and 1923, when Belfast experienced the worst violence in its history. See Bloody Sunday (1921) and The Troubles in Northern Ireland (1920–1922). Killings petered out in 1923 after the signing of the Anglo-Irish Treaty in 1922.

The continuing violence created a climate of fear in the new region, and there was migration across the new border. As well as movement of Protestants from the Free State into Northern Ireland, some Catholics fled south, leaving some of those who remained feeling isolated. Despite the mixed religious affiliation of the old Royal Irish Constabulary and the transfer of many Catholic RIC police officers to the newly formed Royal Ulster Constabulary (1922), northern Catholics did not join the new force in great numbers. Many nationalists came to view the new police force as sectarian, adding to their sense of alienation from the state.

1925–1965 Under successive unionist Prime Ministers from Sir James Craig (later Lord Craigavon) onwards, the unionist establishment practised what is generally considered a policy of discrimination against the nationalist/Catholic minority.

This pattern was firmly established in the case of local government, where gerrymandered ward boundaries rigged local government elections to ensure unionist control of some local councils with nationalist majorities. In a number of cases, most prominently those of the Corporation of Derry, Omagh Urban District, and Fermanagh County Council, ward boundaries were drawn to place as many Catholics as possible into wards with overwhelming nationalist majorities while other wards were created where unionists had small but secure majorities, maximising unionist representation.