During the 7th century BC, Greek sailors reached the lower Danube and sailed upstream, conducting a brisk trade. They were familiar with the whole of the river's lower course and named it the Ister. The Danube later served as the northern boundary of the vast Roman Empire and was called the Danuvius. A Roman fleet patrolled its waters, and the strongholds along its shores were the centres of settlements, among them Vindobona (later Vienna), Aquincum (later Budapest), Singidunum (later Belgrade), and Sexantaprista (later Ruse).
During the Middle Ages the old fortresses continued to play an important role, and new castles such as Werfenstein, built by Charlemagne in the 9th century, were erected. When the Ottoman Empire spread from southeastern to central Europe in the 15th century, the Turks relied upon the string of fortresses along the Danube for defence.
The Habsburg dynasty recognised the navigational potential of the Danube. Maria Theresa, queen of Hungary and Bohemia from 1740 to 1780, founded a department to oversee river navigation, and in 1830 a riverboat made a first trip from Vienna to Budapest, possibly for trading purposes. This trip marked the end of the river's importance as a line of defense and the beginningof its use as a channel of trade.
Regulated navigation on the Danube has been the subject of a number of international agreements. In 1616 an Austro-Turkish treaty was signed in Belgrade under which the Austrians were granted the right to navigate the middle and lower Danube. In 1774, under the Treaty of Kьзьk Kaynarca, Russia was allowed to use the lower Danube.
The Anglo-Austrian and the Russo-Austrian conventions of 1838 and 1840, respectively, promoted free navigation along the entire river, a principle that was more precisely formulated in the Treaty of Paris of 1856, which also set up the first Danubian Commission with the aim of supervising the river as an international waterway.
In 1921 and 1923, final approval ofthe Danube River Statute was granted by Austria, Germany, Yugoslavia, Bulgaria, Romania, Great Britain, Italy, Belgium, Czechoslovakia, Hungary, and Greece. The international Danube Commission was thus established as an authoritative institution with wide powers, including itsown flag, the right to levy taxes, and diplomatic immunity for its members. It controlled navigation from the town of Ulm to the Black Sea and kept navigational equipment in good repair.
During World War II, free international navigation along the course of the river was interrupted by the hostilities, and a consensus concerning the resumption of navigation was not reached until the Danubian Convention of 1948. The new convention provided for the Danubian countries alone to participate in a reconstituted Danube Commission; of these countries, only West Germany did not join the convention.