The Roman general who famously crossed the Rubicon River was Julius Caesar in 49 BC. This act was a pivotal moment in Roman history, as crossing the Rubicon symbolized a declaration of war against the Roman Senate and led to a civil war. Caesar's decision to lead his army into Italy rather than disband them violated Roman law, asserting his defiance against the Senate’s authority. This event ultimately paved the way for Caesar’s rise to power and the eventual transition of Rome from a Republic to an Empire.