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Twenty years ago, the Berlin Wall opened, and events moved so quickly that they seemed inevitable.

But were they?

German unification is a story about how leaders and diplomats moved quickly to transform a political earthquake into a new political and security order for Europe. But it is also the story of how this statecraft responded to and relied on the actions of the German people. U.S. diplomacy was guided by the need to trust the German public as partners in achieving unification.

Secretary of State James Baker and I believed that East Germans would be a driving force for unity. We suspected that the average East German wanted what his or her cousins had in West Germany — and which most East Germans could see on Western TV.

Interestingly, this was not...