History comes back to haunt us. Just over 20 years ago, the then British prime minister Margaret Thatcher told Soviet leader Mikhail Gorbachev: "Britain and western Europe are not interested in the unification of Germany. The words written in the Nato communique may sound different, but disregard them. We do not want the unification of Germany." She went on to say, inaccurately: "I can tell you that this is also the position of the US president." That's according to the Russian record made by one of Gorbachev's closest aides. A British note of the conversation, quoted in a volume of documents just published by Foreign Office historians, adds some fascinating new detail.

This was an act of spectacular disloyalty to an old, faithful, and important Nato ally. It showed a real...