As we celebrate the revolutions of 1989 in eastern Europe – the Polish elections in June, the collapse of the Berlin Wall in November, the liberation of the other countries of eastern Europe, the bloody denouement in Romania – we risk ignoring the thing that made them possible: perestroika, Mikhail Gorbachev’s spectacular revolution in the Soviet Union.

People understood at the time. Adam Michnik, one of the leaders of the Polish anti-Communist opposition, said in July 1989: “Were it not for the ‘perestroika virus’, our [democratic movement] could not have got where it is today.”