The fall of the Roman Republic began in 59 BC with an alliance between three powerful Roman politicians: Julius Caesar, Pompey the Great, and Marcus Licinius Crassus. 
While Caesar was away from Rome leading his army, Pompey gathered political support against Caesar. Civil war erupted when Caesar led his army across the Rubicon River and approached Rome. 
Eventually Caesar defeated Pompey and became the most powerful man in Rome. Caesar's enemies did not want him to end the Roman Republic and become king, so they assassinated him in 44 BC.