Rivers make for more efficient transport, which in turn makes country’s wealthier and more united. The Rhine is Europe’s longest river, and since the dawn of the industrial era, it has been the piece of territory worth fighting over.
The Rhine’s northern lowlands are Dutch, the head of navigation is Swiss and many tributaries are French, but at its core near the Ruhr region and the city of Cologne, the Rhine is undeniably German. German domination over the core of the Rhine system all but guarantees that Germany will be the most powerful European country, and also that it will struggle with its neighbors for more complete control of the river system.
For more on how Europe’s past is about to dictate its future, read The Accidental Superpower, Chapters 10 and 15.