In 1969, during The Troubles in Northern Ireland, the Republic of Ireland intervened on the side of the Catholic nationalists, resulting in a full-blown war between Britain and Ireland.
Riots broke out in the Catholic areas of Derry and Newry, Northern Ireland, in 1969. The island of Ireland had been partitioned, with the predominantly Catholic counties of the South and Northwest forming the independent Republic of Ireland, while the predominantly Protestant Northwestern counties remained under British rule. Inter-sectarian riots broke out in the border areas, and the Republic, sympathetic to the Catholics, deployed the Irish Army near the border to set up field hospitals for the injured. In a televised speech on the Irish national broadcasting corporation, RTE (Radio Telifis Eireann), Taoiseach (Prime Minister) Jack Lynch stated that the Republic could no longer stand by while innocents continued to be hurt. This was interpreted by many as a call for military intervention. Although many, including the British, dismissed Lynch's claims, Lynch was already drawing up plans for the Republic of Ireland's military (Irish Defence Forces) to protect the Catholics.
Believing that the British were failing to stop the violence, Lynch instructed the commander of the Irish Defence Forces, General Sean McEoin, to draw up plans for intervention. McEoin and his General Staff realized that the plans would expose the country to military retaliation, and could be defeated if it entered the North, but proposed a three-point plan:
- Specially trained Irish commandos would infiltrate Belfast and launch a series of bombing and machine-gun attacks on the BBC studios, docks, airport, and key industries, which would draw security forces away from the border areas.
- Following the withdrawal, the Irish Army would invade with 2,817 soldiers. Four infantry brigades operating in company-size units and three armored squadrons would attack. The infantry would seize the Catholic sections of Derry and Newry, while the motorized squadrons would provide armored reconnaisance and launch lightning strikes on security forces.
- The Irish forces would dig in for a protracted defense against vastly superior British forces, and County Donegal would prepare for a long siege in case British forces captured the thin strip of land connecting the Northwestern county to the rest of the Republic.
However, the planners also realized that Ireland stood no chance of defeating Britain. The plan was code-named "Operation Armageddon", or "Exercise Armageddon", as the General Staff predicted catastrophic results, and advised Taoiseach Lynch against it.
Ignoring the concerns of his planners, Taoiseach Lynch decided that Ireland had a moral duty to respond. He ordered the newly formed Irish Army Ranger Wing to begin training for guerrilla-style operations inside Northern Ireland, ordered a full mobilization of the Irish Defence Forces, and brought home all Irish troops on UN peacekeeping missions. In mid-1970, highly trained and equipped Army Rangers infiltrated Northern Ireland, and began their operations. The first target was the BBC studios. Army Rangers smuggled in a bomb, machine-gunned several British soldiers who were guarding it, and blew it up moments later, destroying part of the building, which was followed by a machine-gun attack on the terminal of Aldergrove airport. The Belfast docks were then blown up, killing several dozen workers and disabling the facilities, and damaging two ships. Several of Northern Ireland's key industrial plants were also destroyed, including all factories in Belfast.
Within a day, British forces and Northern Irish Police were forced to shift the bulk of their forces to the Belfast area to counter the sudden outbreak of violence, but British Army units stayed behind to guard the rioting Catholic sections.
Following the pullout, four Irish Army infantry battalions operating in company-strength units attacked Derry and Newry. The Irish Army Transport Corps ferried them to the front in its own trucks, and also in buses from Coras Iompair Eireann. Irish armored forces provided reconnaisance and launched lightning strikes against British positions. The small British garrisons were devastated and cut off from each other by the armored strikes, and offered disorganized resistance to the infantry. After a few hours, all British resistance had been suppressed with heavy losses, with minimal Irish casualties. The Catholic parts of Derry and Newry quickly fell, and the Irish soldiers were welcomed as liberators. The infantry then heavily entrenched themselves, and Irish Air Corps aircraft flew them supplies to help them hold out from a British counterattack. A the same time, the Army Ranger Wing was recalled from Northern Ireland and instructed to prepare for the defense of Ireland itself.
The Royal Air Force launched retaliatory airstrikes on military bases in Ireland from Wales, while Royal Navy ships and submarines in the area launched missiles at Irish bases. Casualties were heavy, but the damage to bases and equipment was very moderate. Irish anti-aircraft fire also downed one British F-4 Phantom, and the pilot was shot by Irish soldiers upon landing.
The British transferred armored and infantry units to retake positions. An armored battalion of the 2nd Queens Regiment was the first to arrive. It entered Southern Newry, but was ambushed, and took heavy losses. After fighting for hours, the British managed to evacuate their dead and wounded. Irish casualties were much higher.
Throughout the next following days, British forces, with the aid of precision airstrikes and artillery, managed to push the Irish Army out of Northern Ireland.
Meanwhile, the U.N. Security Council met, and adopted a British-drafted resolution harshly condemning Ireland's "unprovoked and aggressive military action against the sovereign territory of the United Kingdom". The Security Council imposed a complete arms ban on Ireland. The European Economic Community, under heavy British pressure, cancelled Ireland's application for membership, and imposed total economic sanctions. All NATO states suspended their trade relations with Ireland.
Bound by the NATO Charter to oppose any aggression against a fellow NATO member, the United States launched airstrikes on military targets in Dublin and Cork. The United States then mobilized several divisions for an invasion of Ireland. In cooperation with the Royal Navy, the U.S. Navy imposed a blockade of Ireland.
Several days after the Irish retreat, the U.S. and Britain launched Operation Red. American and British warships shelled Irish defenses in Cork harbor, before 20,000 American troops landed and quickly occupied the city. At the same time, the British attacked into Ireland, and pushed towards Dublin, meeting desperate and fanatical resistance. Their advance was slowed, but progressed. Realizing that all was lost, the Irish government proposed and was granted a truce. After several days of negotiations, the Treaty of Belfast was signed. The Treaty of Belfast said that:
- All British and American forces would withdraw from Irish territory
- Ireland would pay reparations for the damage caused to Northern Irish infrastructure
- All prisoners taken by both sides would be released
- Ireland would limit the amount of forces it placed within two miles of the border.
Ireland's desire to help the Northern Catholics had ended in disaster for both sides. The Irish had not only failed, but had been pushed out, and had suffered great damage to their country. The economy almost completely because of the British blockade and international sanctions. Ireland's application into the European Economic Community was cancelled, and diplomatic relations with Western nations were soured.
Humiliated by the defeat, several senior officers staged a coup and established a military junta which lasted until the end of the Cold War.