The fall of the Berlin Wall was part of a chain reaction started by biologist Janos Vargha and his campaign to stop construction of a dam (Sipa Press/Rex Features)

Twenty years ago the Berlin Wall came down, as the Communist regimes of eastern Europe fell one by one. Who was the first to shake its foundations? Was it cold warrior Ronald Reagan? Or Soviet reformer Mikhail Gorbachev? Well, maybe they had a role. But step forward a tenacious biologist Janos Vargha, whose campaign to halt a dam on the river Danube brought Hungarian hardliners to their knees. When reformers took over in Budapest, their first act was to cancel the dam – and their second was to open the border with Austria. As thousands of Hungarians and East Germans flooded through, the game was up for communism. The wall fell and Europe was transformed.

THE story behind the fall of the Berlin Wall in 1989 began nine years earlier, when Janos Vargha, a biologist from the Hungarian Academy of Sciences began a new career as a writer with a small monthly nature magazine called Buvar. In an early assignment, he went to a beauty spot on the river Danube outside Budapest known as the Danube Bend, the site of an ancient Hungarian capital, to interview local officials about plans for a small park.

It was humdrum stuff – until one official mentioned in passing that this tree-lined curve in the river, a popular picnic spot for Hungarians, was to be drowned by a giant hydroelectric dam being planned in secret by a much-feared state agency known simply as the Water Management. …