At its peak, the Roman Empire spread from Britain to the Middle East, including over 50 million people speaking dozens of different mother tongues, unified by one second language, Latin, one political system, and one powerful cultural ideal. Although there is little real parallel between the Roman Empire and the power the United States wields today, there is one unifying way in which the two states, separated by 2000 years, are surprisingly similar.
The Roman Republic, and the Empire which supplanted it, rose from a sea of squabbling city states, each with their own customs, agenda, and often even a unique language. No one man, or political party, or even culture set out to create an Empire. There was no underlying philosophy of democracy, individual liberty, or social advancement supporting the evolution of Rome from city state to Empire. One war led to another, one political expedient created circumstances where another province had to be admitted to the body politic, one trade war required a public distraction at the other end of the Mediterranean. As much as anything can in history, the Roman Empire just . . . happened.
The United States was created through deliberate effort, the blood of idealistic patriots, and the dreams of men who believed that there were ideas worth dying for. Oh, there was a healthy dose of self-interest, and a great deal of one-thing-leading-to-another involved, but the United States was born in a Declaration of what it was, why it existed, and where it planned to head in the future. Even at the height of its power, Rome had no such philosophical underpinning.
The United States was also enormously fortunate in their isolation. Rome, surrounded by the various civilizations of antiquity, was under constant pressure in multiple directions, much of it backed by military or economic power. From the Revolution to 1865, the United States enjoyed a long period of essentially uninterrupted development and expansion untroubled by large armed conflicts. Several generations of Americans grew up with the ideals of their nation sinking safely into their collective consciousness. These Americans, unlike Rome, also shared a unique but unified culture. Of course, the United States was always a land of immigrants, but during that uninterrupted century the Atlantic and Pacific Oceans kept that immigration to a steady, but manageable trickle. Rome expanded in fits and starts, gulping up whole tribes, cultures, and language groups and making no sustained effort to hammer a unified culture into each of them.
No, it is unreasonable to draw too close a parallel between the United States of today and the Roman Empire of 2000 years ago. However, in one important respect, they both influence or influenced the world of their day similarly through their cultural dominance. Although neither the nations of today nor the scattered civilizations, cultures and tribes of antiquity share the culture of the dominant power, in both cases there is a powerful idea of what it means to be Roman, or American, that is common between the two. In the Empire, being Roman was not where you were born, or what language you spoke at home, or even whether you wore a toga and sandals or trews and wool tunic. "Rome" was an idea of civilization, a notion of a unified ideal of belief, culture, and understanding. A Roman citizen of whatever province was someone who shared an idea which up to that point was probably unique in the world, of something greater than themselves, something called Rome. So too, even in lands where it is despised, does the United States represent far more than the richest and most powerful country on the planet. Being American, as with being Roman in its day, comprises a fundamental belief that there is something great in simply being American. Until Rome, the very idea of "patriotism" did not exist. With the United States, it attains the highest degree of development seen in history.