A South African government memorandum, squirrelled away by a former apartheid operative, has revealed the answer to a 42-year-old mystery: who was behind the devastating 1982 sabotage of the air force of newly independent Zimbabwe? Much more than just another military episode in an unstable era, its violent after-effects hammered yet another nail into the coffin of the fledgling nation's multi-ethnic experiment. And the identification of the spy at the centre of it brings with it exquisite ironies for the Zimbabwean government and its critics - as well as uncomfortable questions for the UK military.

(This is the first in a series of articles on the Thornhill sabotage and its after-effects - Ed)

There was an international furore. It was global news for months. And it was a saga that marked the end of the honeymoon between Robert Mugabe's regime and the West. But the incident that triggered it has remained one of those unsolvable historical mysteries - until now.

In July 1982, saboteurs blew up most of Zimbabwe's fighter aircraft during a nighttime raid on Thornhill, an air force base near the Midlands city of Gweru. Within minutes, the country's fixed-wing airpower had been reduced to smouldering piles of wreckage. The attackers had done their homework: 10 of the country's 12 Hunter fighter jets were stationed at Thornhill, and the base had just received a consignment of British Aerospace Hawk fighter-trainers, purchased at enormous cost to the government's strained fiscus.

The destruction was extensive. Most of the Hunters were ruined, as was one of the Hawks, and the rest were severely damaged. Diplomats reported that "the entire operational fighter force" had been removed from service - and although a few aircraft in storage were resuscitated to fill part of the gap, the air...