Conquer or be Conquered.
There was nothing inevitable about Rome's eventual greatness. It was the most successful of a group of Italian city-states that owed much of their culture to Greek invaders. Rome had succeeded in dominating the nearby cities, but the relationship was uneasy for generations, and there had been rebellions by subjugated neighbours. By 273 BC, however, Rome was mighty enough to challenge Carthage for supremacy in the Mediterranean. It was to fight three long and bloody wars against the Carthaginians and their allies, each time managing to expand Roman influence as a result.
The Punic Wars (from "Poeni", the name the enemy used for themselves) did not always go well for Rome. Hannibal, for example, took a Carthaginian army all the way from Spain, over the Alps and then rampaged across Italy. He seemed unstoppable. But in each war - and even facing a military genius like Hannibal - Rome managed to turn the strategic situation to her advantage. Carthage was eventually entirely crushed, the city was demolished and the fields were sown with salt. Rome was equally merciless with other neighbours when opposed.
The driving force behind Roman expansion was actually Roman politics, in particular the politics of the Senate and of the mob (the people on the streets). Military success in command of Rome's armies could be negotiated into status and power in Roman society and government. Victory also brought spoils and immense wealth. This combination of money and political power made Rome's leaders into aggressive military commanders, ones with an eye to conquest. Bringing new lands into Roman control was a sure fire way (most of the time) of gaining public approval and therefore power. This was Julius Caesar's reason for conquering Gaul. He was supposed to be running his own province not conquering new lands, but there was precious little glory in being a governor. Conquest would, and did, help him gain more power back in Rome, but other generals had done the same before. Where Caesar differed was in the scale of his ambitions. He really did want it all, and for that he was assassinated.
It was left to Caesar's political heir, Octavian (later to call himself Augustus), to eventually become the undisputed master of Rome, but only after a vicious civil war. In 14AD Augustus Caesar died and left the Roman World to his adopted son, Tiberius, in his will. Once the empire could be left to someone like any other possession the Republic was dead, even though Augustus had always been careful to never call himself "emperor". He had always left the Senate a fig leaf of Republican respectability by calling himself the first citizen of Rome. With Tiberius inheriting power, the Empire was born.
