It was Julius Caesar who named the area east of the Rhine as "Germania." Cologne was already a very important Roman city on the Rhine, but only on the west side of the Rhine. The whole vast area east of the Rhine was still unconquered territory in Caesar's day. The victory of the Germanic tribes in the Battle of the Teutoburg Forest (AD 9) prevented annexation by the Roman Empire, although the Roman provinces of Germania Superior and Germania Inferior were established along the west side of the Rhine. Following the Fall of the Western Roman Empire after 400 AD, the Franks conquered the other West Germanic tribes which led to the new early Medieval empire, the Kingdom of the Franks. The Kingdom of the Franks included much of France and western Germany. Out of this vast territory would emerge the Carolingian empire (with both French and German territories) in the 700s and the Holy Roman Empire in the 900s.
A Mighty Fortress: A New History of the German People,
Gordon A. Craig,
TIMELINE OF GERMAN HISTORY: