What were Julius Caesar's contributions to the change from the Roman Republic to the Empire?
Caesar was one of many Roman military leaders who parlayed his success on the battlefield into political power. What was different about his actions, however, was that they ushered in a civil war that ultimately resulted in the rise of his nephew and adopted son Octavian to the position of Emperor of Rome, which spelled the end of the Republic. After his election as consul, Caesar formed alliances with two other powerful politicians, Crassus and Pompey. The three men (called the triumvirate) drew the ire of the Roman aristocracy by their populist appeals, but these also gained them considerable influence.
When Caesar left office, he became governor of provinces in Gaul, and eventually added to his fame by conquering all of Gaul and even making inroads into Britain. His alliance with Pompey and Crassus fell apart, however, and when Pompey secured the Senate's approval of a measure calling on Caesar to return to Rome without his legions, Caesar responded by invading Italy, sparking a civil war. He emerged victorious, and had himself named dictator. While serving in this role (well past the customary one-year term) he initiated a number of reforms that empowered his office as well as currying favor with the common people of Rome.
Eventually, he became dictator for life, and because he also held the title of tribune and consul at various points, he functioned more or less as an emperor. Indeed, he used his power to force the Senate to grant him titles to that effect, though he never officially became a king. His death at the hands of the conspirators paved the way for yet another series of civil wars, first between the conspirators and his lieutenants Marc Antony and Octavian, and then between Antony and Octavian themselves. Octavian, whom Caesar had named as heir to his estate, emerged from these conflicts victorious. With resistance in the Senate crushed, a people hungry for peace and order, and his rivals dead, he completed what his uncle/adopted father had begun.