Marian reforms
After the Marian reforms and throughout the history of Rome's Late Republic, the legions played an important political role. By the 1st century BC, the threat of the legions under a demagogue was recognized. Governors were not allowed to leave their provinces with their legions. When Julius Caesar broke this rule, leaving his province of Gaul and crossing the Rubicon into Italy, he precipitated a constitutional crisis. This crisis and the civil wars which followed brought an end to the Republic and led to the foundation of the Empire under Augustus in 27 BC.