Ancient Rome began as a monarchy. Several of its notable kings were Numa Pompilius, Tullus Hostilius, Ancus Marcius, Servius, and Tarquin. The rule of monarchy ended in a Coup' d' Etat replaced by the Roman Republic in 509 B.C.E. ( the traditionally accepted date) As the Republic financially prospered and geographically expanded the government underwent several transitions. Two of the most important were the twelve tables which secured pleb (lower class) rights in the Senate and the growth of the army.
After the 3rd Punic War Rome was an imperial power. With the conquest of overseas territories came new problems. Referred to as provinces they altered the political landscape of the Republic. In an efoort to address this the republic created new legal status' for people living in the expanded republic. Full Roman Citizenship, Latin Rights, and Italian Allies were among the titles. However, within Rome civil strife was an ever increasing threat. A series of reforms under the Gracchi brothers was a last ditch attempt to salvage the Republic. Unfortunately, for the Republic, the rise of the generals namely Caesar the republics days were numbered.
When Julius Caesar declared himself Dictator for life it was only a matter of time before he would be assassinated 'in the name of the Republic'. This event plunged Rome into civil war for a number of years. As Octavian (Caesar heir and with Caesar's armies) closed in on Antony and Cleopatra, empire was steps away. The Roman Republic ended when Octavian declared himself the 1st Emperor of Rome. (Octavian/Augustus - Pax Romana)
The Roman Empire split between the eastern and western empires by 476 C.E. The western empire was powerless to fight off the Germanic tribes and Rome fell under foreign rule. As for the eastern empire, by 1453 the Ottoman Turks sacked Roman Constantinople, thus bringing an end to the Roman Empire.