On August 27, 1979, Lord Louis Mountbatten is killed when Irish Republican Army (IRA) terrorists detonate a 50-pound bomb hidden on his fishing vessel Shadow V. Mountbatten, a war hero, elder statesman, and second cousin of Queen Elizabeth II, was spending the day with his family in Donegal Bay off Ireland’s northwest coast when the bomb exploded. Three others were killed in the attack, including Mountbatten’s 14-year-old grandson, Nicholas. Later that day, an IRA bombing attack on land killed 18 British paratroopers in County Down, Northern Ireland.

The assassination of Mountbatten was the first blow struck against the British royal family by the IRA during its long terrorist campaign to drive the British out of Northern Ireland and unite it with the Republic of Ireland to the south. The attack hardened the hearts of many Brits against the IRA and convinced Margaret Thatcher’s government to take a hard-line stance against the terrorist organization.

Louis Mountbatten, the son of Prince Louis of Battenberg and a great-grandson of Queen Victoria I, entered the Royal Navy in 1913, when he was in his early teens. He saw service during World War I and at the outbreak of World War II was commander of the 5th destroyer flotilla. His destroyer, the HMS Kelly, was sunk off Crete early in the war. In 1941, he commanded an aircraft carrier, and in 1942 he was named chief of combined operations. From this position, he was appointed supreme Allied commander for Southeast Asia in 1943 and successfully conducted the campaign against Japan that led to the recapture of Burma.

In 1947, he was appointed the last viceroy of India, and he conducted the negotiations that led to independence for India and Pakistan later that year. He held various high naval posts in the 1950s and served as chief of the United Kingdom Defense Staff and chairman of the Chiefs of Staff Committee. Meanwhile, he was made Viscount Mountbatten of Burma and a first earl. He was the uncle of Philip Mountbatten and introduced Philip to the future Queen Elizabeth. He later encouraged the marriage of the two distant cousins and became godfather and mentor to their first born, Charles, Prince of Wales.

Made governor and then lord lieutenant of the Isle of Wight in his retirement, Lord Mountbatten was a respected and beloved member of the royal family. His assassination on August 27, 1979, was perhaps the most shocking of all horrors inflicted by the IRA against the United Kingdom. In addition to his grandson Nicholas, 15-year-old boat hand Paul Maxwell was killed in the attack; the Dowager Lady Brabourne, Nicholas’ grandmother, was also fatally injured. Mountbatten’s grandson Timothy–Nicholas’ twin–was injured; as was his daughter, Lady Brabourne; and the twins’ father, Lord Brabourne. Lord Mountbatten was 79.

The IRA immediately claimed responsibility for the attack, saying it detonated the bomb by remote control from the coast. It also took responsibility for the same-day bombing attack against British troops in County Down, which claimed 18 lives.

IRA member Thomas McMahon was later arrested and convicted of preparing and planting the bomb that destroyed Mountbatten’s boat. A near-legend in the IRA, he was a leader of the IRA’s notorious South Armagh Brigade, which killed more than 100 British soldiers. He was one of the first IRA members to be sent to Libya to train with detonators and timing devices and was an expert in explosives. Authorities believe the Mountbatten assassination was the work of many people, but McMahon was the only individual convicted. Sentenced to life in prison, he was released in 1998 along with other IRA and Unionist terrorists under a controversial provision of the Good Friday Agreement, Northern Ireland’s peace deal. McMahon claimed he had turned his back on the IRA and was becoming a carpenter.