For a long time, the Rhine river remained an important threshold for the ancient Romans. The Roman empire stretched to the southeast of the river, with Germanian tribes occupying lands on either side of the northern parts of the Rhine. Military expeditions crossing the Rhine were infrequent, but gained considerable notoriety due to the fierce reputation of the native tribes beyond the river; Julius Caesar is the first known Roman to have crossed the Rhine. Significant fortifications were built by order of Emperor Caligula some 90 years later, and shortly afterwards the Empire began its expansion onto the eastern shore of the Rhine. Although the lands gained during that time were subsequently lost around 260 AD, the lands to the west of the Rhine remained under control of the Roman empire until the Frankish tribes invaded and conquered much of what is now western Europe in the 6th century.
The Holy Roman Empire, consisting mostly of lands surrounding the Rhine, lasted from the 9th century to the early 19th; it ceased to exist in 1806 after a major military defeat by the expanding French empire of Napoleon I. Following the treaty of Pressburg, Napoleon formed the Confederation of the Rhine, a satellite state with Napoleon as its "Protector." The state grew steadily and absorbed lands surrounding the river; however, it didn't survive Napoleon's defeat in 1813.
Following the Franco-Prussian war and the treaty of Versailles in 1871, the Rhine and its surrounding lands became a part of the freshly unified German state. Those lands were eventually regained by France following Germany's defeat in World War I, and a steady build-up of military personnel and fortified defensive systems on the shores of the Rhine followed shortly afterwards. A major part of the Maginot Line, a system of French fortifications, followed the western shore of southern part of Rhine, while its German counterpart, the Siegfried Line, was formed on the opposing shore. Those fortifications, as well as the river itself with its countless, bridges played a major part in World War II; the Siegfried Line in particular was an important threshold as the Allied forces made their final push towards invading Germany.
