Athenian democracy, Roman Republic, Roman Empire

the Empire was virtually a Dictatorship and the republic was a democracy

Julius Caesar is the military leader and the dictator who transformed the roman republic into an roman empire. He assumed total power after transforming the republic into the empire.

The Roman Republic was ended by the takeover of the senate by Ceaser. He essentially became the supreme ruler, or emporer. So essentially, the Roman Republic was replaced by the Roman Empire.

The Holy Roman Empire had nothing to do with the Romans. It was a medieval institution centred around Germany. The Roman republic became an empire by expansion both before and after the Punic wars.