The gruesome 1979 IRA assassination of a beloved British royal—which took place the same day as a deadly coordinated attack on British troops—led to outrage, heartbreak and a heightening of “The Troubles,” the decades-long Northern Ireland conflict.

The Provisional Irish Republican Army claimed responsibility for the August 27, 1979, murder of Lord Louis Mountbatten, 79, Earl of Burma, great-grandson of Queen Victoria, second cousin of Queen Elizabeth II and great-uncle of King Charles III. The World War II hero and last viceroy of India was aboard his 29-foot Shadow V fishing boat with six others near his summer home in northwest Ireland the morning of the attack.

August 27, 1979, a Bank holiday, had dawned sunny, following days of rain. “Dickie” Mountbatten and some of his family who had been staying at their holiday home, Classibawn Castle near the Village of Cliffoney, County Sligo in the Republic of Ireland, decided to take an outing on their boat to take in the good weather.

Fifteen minutes after setting sail, a planted bomb was activated by two members of the Provisional IRA, a paramilitary group of Irish nationalists who waged a terror campaign to drive British forces from Northern Ireland to create a united, independent nation. Known as "the Troubles," the conflict raged for 25 years before IRA and loyalist ceasefires were initiated. By 1998, the year the Good Friday or Belfast Agreement settled the conflict, more than 3,600 people had died.

“The boat was there one minute and the next minute it was like a lot of matchsticks floating on the water,” a witness told the New York Times.

The party of seven aboard the Shadow included Mountbatten, his daughter Patricia, her husband, Lord John Brabourne, their 14-year-old twins, Timothy and Nicholas, and Lord Brabourne’s mother, the dowager Lady Doreen Brabourne. Paul Maxwell, 15, a friend of the family who worked on the boat, was also on board. Mountbatten, Nicholas Brabourne and Maxwell were killed immediately. Lady Brabourne died the next day and the others survived serious injuries.

“Fifty pounds of gelignite exploded, sending showers of timber, metal, cushions, lifejackets and shoes into the air,” Andrew Lownie, author of The Mountbattens: Their Lives and Loves, wrote for the BBC. “Then, there was a deadly silence.”

But the explosion wasn’t the only carnage that day. Later that afternoon, 18 British soldiers were killed near the Irish border at Warrenpoint in an IRA bombing ambush. “It was the single heaviest death toll for the British Army in the 10 years since it was sent in to quell fighting between Roman Catholic and Protestant militants,” according to the Times.

Mountbatten was both a sentimental and symbolic target. “He was one of the most respected members of the royal family and was serving as mentor to [then] Prince Charles,” says Jeffrey Lewis, lecturer in the International Studies Program at Ohio State University.

Mountbatten was also an easy target. The bomb had been placed in his unguarded boat the night before his murder. He had been vacationing in the Irish town of Mullaghmore throughout the 1970s and had refused security detail, despite repeated threats from the Provisional IRA to assassinate him. Mountbatten had declared, “Who the hell would want to kill an old man anyway?”

Brendan O’Leary, a political science professor at the University of Pennsylvania and author of A Treatise on Northern Ireland, notes that while Mountbatten could not have predicted the IRA would plant and trigger a bomb on his boat, he had been lax about his own security.

“He had been supreme allied commander in southeast Asia, and was said to have been the youngest admiral in the history of the Navy,” he says. “He was also known as the last viceroy of India, who had overseen its partition. He was therefore a very prominent public figure, but a retired man of 79, who played no role in the British security forces in Northern Ireland, and who regularly holidayed in Ireland, could not be described as a legitimate war target.”

Timothy White, a political science professor at Xavier University who teaches courses on Irish culture and politics, adds that by assassinating one of the most beloved members of the royal family, the IRA hoped to convince the British to leave Northern Ireland and allow Northern Ireland to join the Irish republic.

“By killing such a high-profile and public figure, the IRA wanted everyone in England to feel scared of the potential of the IRA to terrorize the British population,” he says.

A statement from the Provisional IRA claimed immediate responsibility for Mountbatten’s “execution,” calling it “a discriminate act to bring to the attention of the English people the continuing occupation of our country. … The death of Lord Mountbatten and tributes paid to him will be seen in contrast to the apathy of the British Government and English people to the deaths of over 300 British soldiers and the deaths of Irish men, women and children at the hands of their forces.”

Provisional IRA bombmaker Thomas McMahon, 31, was found guilty of the Mountbatten attack and was sentenced to life. IRA activist Francis McGirl, 24, was acquitted. McMahon was released from prison after serving 19 years as part of the Good Friday Agreement.

The August 27 attacks led to widespread fear and outrage in the region, according to Lewis.

“The indiscriminate nature of the attack led many to condemn the IRA as savage and cowardly,” he says. “At the same time, the sophistication of the bomb—it was detonated by radio remote control—coupled with the Warrenpoint ambush suggested that the IRA was becoming more dangerous and capable. This combination—savagery coupled with tactical competence—was very unsettling.”

Among the English, the reaction in Parliament, newspapers and newscasts, was outrage. O’Leary, who grew up in Northern Ireland and Sudan, points out that Mountbatten had been a war hero and important mentor to King Charles III. His killing and that of his grandchild and his daughter’s mother-in-law ”were regarded as especially outrageous.” In Ireland, he adds, there was outrage that a guest had been killed, as well as children and a woman who had no public or security force connections.

“Ulster Unionists called for increased security, and called the Republic of Ireland a safe haven for terrorists—in fact, the Irish police were able to identify and convict the organizer of the bombing of Mountbatten’s boat with forensic evidence,” O’Leary says. He adds that among those who were sympathetic to the IRA’s causes, more supported the attack of soldiers from the Parachute Regiment, which had been behind the massacre of civilians on Bloody Sunday in January 1972, than the murder of an elderly, retired grandfather and his family.

Margaret Thatcher, elected prime minister just before the assassination, saw the IRA as a criminal, rather than political, organization. She responded by withdrawing political rights associated with prisoner of war status for IRA prisoners. The IRA responded in turn with a hunger strike. The leader of the hunger strike, Irish nationalist Bobby Sands, was then elected to British Parliament but would die in prison from his hunger strike on May 5, 1981. Ultimately, White says, the murder of Mountbatten and his family signaled a raw, dark period ahead for England and Northern Ireland.