For much of its history the Danube has been a disappointment. It looks so tempting on the map but, far from being a natural motorway for trade and ideas, its sheer awkwardness has thwarted generations of visionaries, engineers, soldiers and dictators. Freezing up, expanding into baffling flood-plains, racing through narrow defiles and randomly scattered with dangerous islands and hidden rocks, it has at best tended to function only for fishermen and the most local trade.