Growing tension and resentment in Ireland over British rule and the question of Irish independence set in motion a chain of events that came to be known as the Irish Revolution. Conflicts between nationalists, who wanted a completely independent Irish republic, and unionists, who wished to remain under British control, led to the establishment of armed paramilitary groups in both areas of the island. The threat of civil war between the factions over the question of Irish independence was imminent. This crisis was temporarily averted, however, by the outbreak of World War I (1914-1918), and representatives of both groups supported the British war effort against Germany. However, one splinter group of the nationalist faction refused to join the war effort. Instead, they initiated the Easter Rebellion of 1916, capturing the city of Dublin and declaring the independence of Ireland. The rebellion was unsuccessful, primarily due to limited support from the Irish people. However, Britain’s severe reaction, including the execution of 15 Irish nationalist leaders, outraged the Irish population and set the stage for the nationalist group Sinn Fein to become the dominant political party in Ireland. Sinn Fein had been organized several years before the uprising by Arthur Griffith, a Dublin journalist. Sinn Fein now called for Ireland, including the northern areas, to become a republic independent of the United Kingdom. In the 1918 election, Sinn Fein candidates won 73 of the 106 seats allotted to Ireland in the British Parliament.

Ireland, lying to the west of Britain, has always been to some extent cut off by it from direct contact with other European countries, especially those from Sweden to the Rhine River. Readier access has been through France, Spain, and Portugal and even Norway and Iceland. Internally, the four ecclesiastical provinces into which Ireland was divided in the 12th century realistically denoted the main natural divisions of the country. Of these, the north had in the earliest times been culturally connected with Scotland, the east with Roman Britain and Wales, the south with Wales and France, and the southwest and west with France and Spain. In later times, despite political changes, these associations continued in greater or lesser degree.

On Easter Monday, April 18, 1949, the anniversary of the Easter Rebellion, Eire became the Republic of Ireland, formally free of allegiance to the British crown and no longer a member of the Commonwealth of Nations. In the following month, the British Parliament confirmed the status of Northern Ireland as part of the United Kingdom until its own parliament chose otherwise. It allowed Ireland to retain the economic benefits of Commonwealth membership, and it extended to Irish citizens resident in the United Kingdom the same rights as British citizens. Ireland granted British citizens residing in the republic similar benefits, not including political rights. The republic became a member of the United Nations on December 14, 1955. It declined to join the North Atlantic Treaty Organization (NATO), however, since this would have entailed entering into an alliance with the United Kingdom, which retained possession of Northern Ireland. .