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Former German Chancellor Helmut Kohl has never forgotten the hostility he faced at a European meeting on December 8, 1989. Ten days earlier he had unveiled a 10-point-plan for German reunification and been met with the blatant skepticism of Europe's leaders. In his memoirs, the former chancellor has described how British Prime Minister Margaret Thatcher famously told the heads of state when they were gathered for dinner: "We beat the Germans twice, and now they're back."

It's no secret that Thatcher was a bitter opponent of German reunification. But new documents released Thursday by the Foreign and Commonwealth Office show how she insisted that her...