Danube
The Danube flows through many cities, including four national capitals (shown below in bold), more than any other river in the world. Ordered from the source to the mouth they are:
The Danube is navigable by ocean ships from the Black Sea to Brăila in Romania and by river ships to Kelheim, Bavaria, Germany; smaller craft can navigate further upstream to Ulm, Württemberg, Germany. About 60 of its tributaries are also navigable.
Battle of Neuburg (1800)
The French war goal, to occupy Vienna and force the Habsburgs to accept and comply with peace terms established in 1798, required a double-pronged invasion through northern Italy, which First Consul Napoleon commanded, and through southern Germany, a campaign that fell to Moreau. To secure access into Bavaria and, eventually, to Vienna, the French needed to control the Danube riverway. This was not a new tactic: The stretch of river between Ulm and Neuburg had been the site of major battles of the Thirty Years' War and War of the Spanish Succession. Between Ulm and Ingolstadt, the Danube grows significantly in volume, making it a wide and swift waterway. The Iller joins the Danube at Ulm, dumping massive amounts of water into the stream; at Donauwörth, the Lech enters the Danube. Neuburg, the first significant city on the river after Donauwörth, had been the family seat of the princes of Pfalz-Neuburg; taking it from a princely family of the Holy Roman Empire would be a blow to the morale and prestige of the Habsburgs, whose role it was to protect the small princely domains. Control of the bridges and passages between Ulm and Donauwörth, Neuburg, then Ingolstadt offered an advantage of both transport and prestige.