it happened on the way to the forum: caput mundi or manifest destiny

The fledging republic of Rome was one of many Italian tribes with aspirations of expansion, security and trade—and probably would have been remained just another ambitious but obscure and forgettable clan without very original stories, had not they suffered a crushing defeat early on that tested their mettle and resilience and will for rebuilding and reform. The Roman Republic and Empire endured in the West for nearly a thousand years before the Gothic invasion, which precipitated its ultimate collapse. Imperium and trade were vehicles of culture—as the Latin language spread to ends of the earth, so too did the customs and beliefs that Romans adopted, with cults and dedications to Egyptian Ra and Osiris translated to Britain. We have the Gauls—specifically the Senones who crossed the Alps and occupied lands in northern Italy under the leadership of Brennus—to thank for kicking sand at the Romans. Some believe that the attack on Rome was a conspiracy between the Greek leaders of Syracuse in Sicily and Sparta—using the Gauls as a proxy force, to defeat Rome and gain control of the rest of the Italian peninsula. That is not how the Romans tell it—however, and as all records were destroyed it the siege, we’ll have to rely on the version of the vanquished. 
The Senones had crossed the Apennines and began raiding the Etruscan cities, including Siena. The Etruscan were former enemies of Rome but were now under an uneasy yoke of allegiance after surrendering to Rome. Those former battles probably made this flank of Roman territory vulnerable to incursion. In any case, Rome sent an embassy to negotiate a peace. When talks broke down, however, fighting resumed and the diplomats (against the standards of statecraft and the unwritten ius gentium, the Law of Nations, which addressed such conduct) joined in the scuffle. Outraged, Brennus demanded justice for this transgression and the citizens of Rome responded by appointing the diplomats as Military Tribunes. For this further insult, the Senones resolved to march south towards Rome. The Roman defensive lines fell rather quickly and panicked, the Romans retreated to the citadel—leaving the gates open and the city vulnerable. Not quite believing that success came so easily, the Senones waited a full day before entering the city—unconvinced that it was not a trap of some sort. With all the population holed-up, the invaders decided that they would starve the Romans out and prevail by attrition. The city was looted and with no one to respond, blazes engulfed much of the city. The Senones, however, were not well equipped for this waiting-game either and not accustomed to the hot weather and probably just wanted to go home at this point. Eventually a deal was struck, allowing the Romans to purchase peace at the high price of one thousand pounds (libra or libra pondo—a pound by weight, and hence the abbreviation lb and symbol £) of gold. As the Romans were emptying out there coffers, they noticed that the balance was weighted in Brennus’ favour—taking more gold to make a pound than it should have. When the Romans objected, Brennus slammed down his belt and sword, tipping the scales even more to his advantage, saying “Woe to the vanquished”—“Vae victis.” Dishonoured and their city in ruins, the Romans thought about disbanding and abandoning their capital and starting over, but the Senate rallied the people to rebuild—which gave us the jumbled, crowded Eternal City of today, being that there was little time for civic-planning as opposed to the orderly garrisons and outposts that the legions built as exhibitions began again almost immediately, and adopted new policies towards expansion and empire, having learnt from this experience—also adopting the fighting style and weapons of the Gauls. Of course, Rome would come to one day rule over Gaul and much of the broader expanses influenced by Celtic peoples, rebuilt and refurbished for conquest.