The first Roman general to cross the Rubicon River was Julius Caesar in 49 BC. This act was laden with significance, as crossing the Rubicon was considered a declaration of war against the Roman Senate and a direct challenge to Pompey, whom Caesar opposed. The phrase "crossing the Rubicon" has since entered the lexicon as a metaphor for committing to a decisive and irreversible action. Caesar's crossing initiated a civil war that would ultimately lead to his rise as dictator of Rome, fundamentally changing the course of Roman history. It marked the end of the Roman Republic and the beginning of imperial rule.