Open this photo in gallery: The AmaWaterways Danube River Cruise begins in Budapest, Hungary, above, and finishes in Vilshofen, Germany.Ama Waterways The Volga is longer, the Rhone is more important economically, the Rhine is fairy-tale prettier. But the Danube is the cruise to choose if you want to understand how Europe became what it is. This is the waterway that links Europe to Asia, that marked the northern border of the Roman Empire and the western frontier of the vast Ottoman sultanate. Pliny and Ptolomey wrote about the Danube, and so did Ovid. In September, I travelled the storied river...