When and why did the Rhine become a Roman boundary?

The Rhine became a Roman boundary due to a series of military campaigns by Julius Caesar in the first century BC. These campaigns resulted in the incorporation of Gaul into the Roman Republic, with the Rhine serving as a natural border between Roman territory and the Germanic tribes to the east. The Rhine remained an important boundary for the Roman Empire for several centuries, with a series of forts and fortifications being built along its banks to defend the frontier against potential incursions from the Germanic tribes.