No movement of any consequence took place for more than twenty years after Cæsar's last departure from the banks of the Rhine. The Romans, having secured their possession of Gaul, now turned their attention to the subjugation of the Celtic tribes inhabiting the Alps and the lowlands south of the Danube, from the Lake of Constance to Vienna. This work had also been begun by Cæsar: it was continued by the Emperor Augustus, whose step-sons, Tiberius and Drusus, finally overcame the desperate resistance of the native tribes. In the year 15 B. C. the Danube became the boundary between Rome and Germany on the south, as the Rhine already was on the west. The Roman provinces of Rhætia, Noricum and Pannonia were formed out of the conquered territory. Augustus now sent Drusus, with a large army, to the Rhine, instructing him to undertake a campaign against the independent German tribes. It does not appear that the latter had given any recent occasion for this hostile movement: the Emperor's design was probably to extend the dominions of Rome to the North Sea and the Baltic. Drusus built a large fleet on the Rhine, descended that river nearly to its mouth, cut a canal for his vessels to a lake which is now the Zuyder Zee, and thus entered the North Sea. It was a bold undertaking, but did not succeed. He reached the mouth of the river Ems with his fleet, when the weather became so tempestuous that he was obliged to return. The next year, 11 B. C., he made an expedition into the land of the Sicambrians, during which his situation was often hazardous; but he succeeded in penetrating rather more than a hundred miles to the eastward of the Rhine, and establishing--not far from where the city of Paderborn now stands--a fortress called Aliso, which became a base for later operations against the German tribes. He next set about building a series of fortresses, fifty in number, along the western bank of the Rhine. Around the most important of these, towns immediately sprang up, and thus were laid the foundations of the cities of Strasburg, Mayence, Coblenz, Cologne, and many smaller places. [Sidenote: 9 B. C.] In the year 9 B. C. Drusus marched again into Germany. He defeated the Chatti in several bloody battles, crossed the passes of the Thuringian Forest, and forced his way through the land of the Cherusci (the Hartz region) to the Elbe. The legend says that he there encountered a German prophetess, who threatened him with coming evil, whereupon he turned about and retraced his way towards the Rhine. He died, however, during the march, and his dejected army had great difficulty in reaching the safe line of their fortresses.