Celtic warfare
The "Gallic Wars" were a series of military campaigns waged by the Roman proconsul Julius Caesar against Gallic tribes, lasting from 58 BC to 51 BC. The Romans would also raid Britannia and Germania, but these expeditions never developed into full-scale invasions. The Gallic Wars culminated in the decisive Battle of Alesia in 52 BC, in which a complete Roman victory resulted in the expansion of the Roman Republic over the whole of Gaul. The wars paved the way for Caesar to become the sole ruler of the Roman Republic.
Constitutional reforms of Augustus
Julius Caesar campaigned in Gaul from 59 BC to 49 BC, which granted him unmatched military power and popularity with the people of Rome. With the Gallic Wars concluded, the Senate ordered Caesar to lay down his military command and return to Rome as a "privatus" ("private citizen"). Caesar refused, and marked his defiance in 49 BC by crossing the Rubicon River at the head of his army, leaving his province and illegally entering Roman territory under arms. Civil war resulted, from which Caesar emerged as the unrivaled ruler of Rome.