CAN one man change history? Historians have long answered “No” to that question. But the story of the MI5 agent known only as Robert — who went way beyond his brief in secret negotiations with the IRA, and thereby opened the door to peace in Northern Ireland — might give us pause for thought.

The veteran reporter Peter Taylor spent nearly four decades tracking down Robert to piece together the final details of his secret 1993 meeting with the IRA’s chief of staff, Martin McGuinness. Last week, Mr Taylor finally broadcast an extraordinary interview with Robert (The MI5 Spy and the IRA: Operation Chiffon, available on BBC iPlayer), which laid bare the clandestine encounter and its far-reaching implications.

Even when I was at school, the Great Man theory of history was considered dead and buried. In the 19th century, Thomas Carlyle had insisted that “The History of the world is but the Biography of great men.” Generations of children had been taught history that way, despite Carlyle’s idea having been swiftly debunked by Herbert Spencer, whose Social Darwinism argued that society made men, and men did not make society. Tolstoy saw individuals as “history’s slaves”; Hegel said that great men merely uncovered an inevitable future; and Marx went the full hog with historical materialism.

Yet it is hard not to think of how things might have turned out differently in our times without Winston Churchill, Mahatma Gandhi, Mikhail Gorbachev, and Nelson Mandela.

And it need not be a great or famous person. On 26 September 1983, Lt. Col. Stanislav Petrov was at his post at the Soviet early-warning network when the alarm went to say that an intercontinental ballistic missile was heading toward the Soviet Union from the United States.

The Soviet protocol required an immediate “launch on warning” compulsory nuclear counter-attack against the US. Petrov dismissed the detection as computer error — and held his nerve when the computers identified four more missiles. A real attack, he judged, would have involved hundreds of missiles to disable a Soviet counterattack. He broke the rules, and changed history.

Three days before Robert’s scheduled meeting with the IRA, two bombs went off in Warrington, killing two children. Robert was told by his bosses not to attend. He ignored them. Though he was appalled by the bombing, he saw it as in line with the Republican strategy whereby Sinn Fein talked peace while the IRA carried on killing.

Robert had written dozens of reports on how the IRA’s top leaders wanted peace but wanted a firm sign that Britain sincerely wanted to negotiate. So he told McGuinness: “The final solution is union. This island will be as one.”

When MI5 found out what he had done, Robert had to resign. Five years later, when the Good Friday Agreement was signed, his old bosses rang to thank him. His highly contentious and unauthorised words broke the logjam, so that, in 1998, unionists were able to feel that the Union was safe, while nationalists could believe that demographic trends would ultimately bring consent for reunification from most people in Northern Ireland.

It may generally be economic, political, and social forces that shape our history. But sometimes it takes the actions of an individual to channel the forces of their age to produce change.