So how does the United States of America match up with all these behemoths?

Of all the empires that arose and thrived on the face of this earth, which were the five most powerful? And how is it even possible to select five empires from among the hundreds that have flourished over the past five thousand years? Truth be told, any formulation of the “five most powerful empires” will always be subjective, because all empires were glorious and influential in their own ways.

(This first appeared several years ago.)

But there are some empires that were simply so powerful, large, and influential over the grand sweep of history that they deserve to be called the greatest, no matter the criteria. The reader may note that I left out empires from China and India. While I would be the first to acknowledge the importance and legacy of empires from these regions, the overall global legacy of empires from these regions tends to be regional.

The (First) Persian Empire

The Achaemenid Persian Empire was founded by Cyrus the Great around 550 B.C.E., who went by the title of King of Kings (Shahanshah). Although the Persian Empire came to an inglorious end at the hands of Alexander the Great in 330 B.C.E., it had a lasting legacy on the subsequent development of world civilizations and future empires. Indeed, the Persian Empire was a pivotal empire because it was the first true empire that set the standard of what it meant to be an empire for future ones.

The Persian Empire existed at a unique time in history, when most of the oikumene, or civilized, settled, populated world was concentrated in or near the Middle East. As a result, the Persian Empire, which dominated most of the Middle East, ruled over a greater percentage of the world’s population than any other empire in history. Indeed, in 480 B.C.E., the empire had a population of approximately 49.4 million people, which was 44 percent of the global population at that time. The Persian Empire was the first empire to connect multiple world regions, including the Middle East, North Africa, Central Asia, India, Europe, and the Mediterranean world. It jumpstarted the concept of empires in places like Greece and India.

Such a large empire could only have been put together by military might, and the Persian Empire’s military achievements were significant, though they are often forgotten by its sudden demise at the hands of Alexander’s armies. Various Persian campaigns succeeded at subjugating most of the world’s advanced civilizations at the time including the Babylonians, Lydians, Egyptians, and the northwestern Hindu region of Gandhara, in today’s Pakistan. It should not be forgotten that, notwithstanding exaggeration and misinterpretation, the Persians believed that they achieved their goals in Greece and that more Greeks lived in the empire than not. The Persian Empire ushered in a period of harmony and peace in the Middle East for two hundred years, a feat that has seldom been replicated.

Recommended: What Will the Sixth-Generation Jet Fighter Look Like?

Recommended: Imagine a U.S. Air Force That Never Built the B-52 Bomber

Recommended: Russia's Next Big Military Sale - To Mexico?

Recommended: Would China Really Invade Taiwan?

The Persian Empire’s legacy to the world in terms of imperial ideas include the use of a network of roads, a postal system, a single language for administration (Imperial Aramaic), autonomy for various ethnicities, and a bureaucracy. The Persian religion, Zoroastrianism, influenced the development of key concepts like free will and heaven and hell in Abrahamic religions through Judaism.

The Roman Empire

This one should be obvious. The Roman Empire has long been the empire par excellence for the Western world. But its importance is not the product of Western bias: the Roman Empire was truly one of history’s greatest empires. The Romans displayed the awesome ability to conquer and hold large swathes of territory for hundreds or even thousands of years, if the Eastern Roman Empire (Byzantine Empire) is accounted for. But it was not held together by brute force alone; once conquered, people aspired to become Roman, which meant participating in a sophisticated, urbane, classical culture.

Several important features of the modern world are the result of the Roman Empire. The Romans took over and expanded upon the Hellenistic (Greek) culture, passing down Greek architecture, philosophy and science to future generations. Later, the Roman embrace of Christianity helped elevate that religion from a minor cult to one of the world’s great religions.

Roman Law also influenced all subsequent legal systems in the West. Roman institutions also helped inspire the governance systems of modern democracies. Despite Greece’s reputation as the “birthplace of democracy,” the American Founding Fathers were primarily influenced by British and Roman practices. In fact, many of them frequently spoke of their distaste for the Athenian experiment in democracy and their admiration for the Roman form of mixed government, where monarchical, aristocratic, and democratic elements shared power. The American political system— with its separate branches of government— approximate this Roman institutional division. Once the Roman Republic transitioned into the Roman Empire, the idea and majesty of Caesar served as an inspiration for future rulers.

The Romans were a tenacious people. They were able to bounce back from numerous setbacks against improbable odds to pull together and defeat their enemies. Though the Carthaginian general Hannibal almost destroyed the Romans after the Battle of Cannae in 216 B.C.E., the Romans were able to land an army at Carthage to defeat it a mere fourteen years later. The Roman legions were militarily dominant for centuries, enabling Rome to rule over nearly all other civilized peoples in the Mediterranean and Near East except the Persians for hundreds of years and facing only minor raids by disorganized tribes. When the empire did collapse, it was due more to continued crisis and civil war rather than its invasion by Germanic tribes. And the Eastern Empire lasted until 1453 C.E., giving the political history of the Roman state a whopping two millennia span.

The Caliphate

The Arab Empire, also known as the Caliphate, was a political entity founded by the Muslim Prophet Muhammad that encompassed most of Arabia by the time of his death in 632 C.E. It is more reasonable to call this the Arab Empire rather than the Muslim Empire because while Islam originated and spread because of this empire, there were many subsequent empires that were legally Muslim or ruled by Muslims but were not Arab.

Muhammad was succeeded by the four Rightly-Guided Caliphs (“successors”) who were selected by consensus and acclimation (though not undisputed) until 661 C.E. The hereditary Umayyad Caliphate then ruled until 750 C.E., followed by the Abbasid Caliphate, though conquests had ended by this point. The Arab Empire effectively ended around 900 C.E., although the Abbasids maintained their religious role as figurehead Caliphs in Baghdad until the destruction of that city by the Mongols in 1258 C.E. After 900 C.E., the empire began to crumble politically with the rise of rival dynasties, many of them Turkic and Persian in origin, as well as rival Caliphates in Spain and Egypt.

Nonetheless, in its own time the Arab Empire was extraordinary, both because of its military successes, and because of its legacy. It is amazing that a loosely organized, tribal people on the fringes of world civilization defeated the Byzantine Empire and overthrew the Sassanid Persian Empire, both of whose populations and resource bases dwarfed the Arabian Desert. The Arab conquests are a good example of how ideological zeal can sometimes make up for technological and organizational deficiencies, and Arab generals from this period deserve to be ranked among the world’s greatest military geniuses, especially the third Caliph Omar, who conquered the region from Egypt to Persia in ten years. In a hundred years, the Arab Empire grew to be several times larger than the Roman Empire at its height.

Because of its location, the Arab Empire, like the Persian Empire before it, connected the other centers of world civilization in Africa, Europe, Central Asia, India and China. As a result, goods and knowledge from all these regions were able to mix for the first time, giving rise to new concepts like algebra.

The ultimate legacy of the Arab Empire, of course, is the religion of Islam, followed by more than a billion people today.

The Mongol Empire

The Mongol Empire was another empire that originated on the periphery, and against all odds, defeated enemies much more powerful and populous than it. It was the world’s largest contiguous land empire, one that struck terror into all its enemies. Founded by the Mongol warlord Temujin, who assumed the title of Genghis Khan in 1206 C.E., the Mongol Empire first grew by picking off parts of China, as many previous steppe tribes had done.

But the defining moment of the Mongol Empire was when its ambassadors were killed by leaders of the neighboring Khwarazmian Empire, which included Iran, Afghanistan, and Central Asia. This was perceived as a grievous affront to the Great Khan and the subsequent Mongol revenge completely wrecked Central Asia and ended its Golden Age. Combined with the subsequent establishment of European sea routes that bypassed the Silk Road, the Mongol Invasions spelled the doom of Central Asia as an important region.