What is the significance of Caesar crossing the Rubicon River?

Caesar's crossing of the Rubicon River was significant because it was deemed illegal (Roman law) for any general to cross this body of water. Governors of Roman provinces were not allowed to bring any part of their army into Italy. If they did, they forfeited their right to rule (even in their own province). After a four-year struggle, Caesar emerged victorious over Pompey and took command of the region now recognized as the Roman Empire.