To a remarkable extent, world history from the 1860s to after the Second World War has been dominated by the German Problem. What is, or was, that problem? Its international aspect can most easily be summarized by a question: Why, since the 1860s, has Germany been so often at war with its neighbors? There is a domestic aspect as well: What made a Nazi regime possible in so advanced and civilized a country?

Like the German Problem itself, the theories that explain it have their international and domestic components. Internationally, a united Germany is often said to have been too big and dynamic for any stable European state system. Inevitably, such a Germany threatened the political independence and economic well-being of its neighbors. Germany's dynamic expansiveness, in turn, is frequently said to have stemmed from the internal character of the German nation – the political institutions, culture, and economic and social systems that evolved during the nineteenth and twentieth centuries. This character is thought not only to have made Germany uncommonly aggressive abroad, but also particularly susceptible to totalitarianism at home.

For many people, these theories lead to a single practical conclusion: Whenever unified into one state, Germans become a menance at home and abroad. And this conclusion, shared by the postwar leaderships of all Germany's neighbors, leads to an obvious prescription: To keep Europe safe, and the German people safe from themselves, Germany must be firmly contained by alert and superior power. Had the lesson only been learned and applied earlier, it is frequently said, the world would have been spared at least the second of this century's general wars.