Legend has it that Romulus and Remus—twin brothers who were also demi-gods—founded Rome on the River Tiber in 753 B.C. Over the next eight and a half centuries, it grew from a small town of pig farmers into a vast empire that stretched from England to Egypt and completely surrounded the Mediterranean Sea.

The Roman Empire conquered these lands by attacking them with unmatched military strength, and it held onto them by letting them govern themselves.

Rome’s desire to expand had deep historical roots, says Edward J. Watts, a professor of history at the University of California, San Diego, and author of Mortal Republic: How Rome Fell Into Tyranny.

“There’s a tradition going back to basically Roman prehistory, mythological history, where they talk about the expansion of the city under the kings,” he says. “Marcius is one of the early Roman kings [from 642 to 617 B.C.], and he’s said to actually have engaged in expansion and extended the city to incorporate other hills. So the idea of them expanding is always deep in the historical DNA of the republic, and even the monarchy before the republic.”

Even so, Rome was still relatively small by the time it transitioned from a kingdom to a republic in 509 B.C. The republic’s first significant expansion came in 396 B.C., when Rome defeated and captured the Etruscan city of Veii. Instead of destroying Veii, the classicist Mary Beard argues the Romans largely let the city continue operating as it had before, only under Roman control and with the understanding that Rome could conscript free men for the Roman army.

The conquest of Veii was “a big turning point for [the Romans] because they take over a territory that’s half the size of the territory they already have,” Watts says. Over the next two-and-a-half centuries, Rome spread throughout the Italian Peninsula by conquering territories and either making them independent allies or extending Roman citizenship.

“The absorption of Italy was actually an absorption; it wasn’t supposed to be a colonial regime,” he says. Later, in the first century B.C., it extended Roman citizenship to all free people. Still, it never extended citizenship to the many enslaved people in Italy obtained through trade, piracy, wars and other means.

This strategy of absorption changed as Rome conquered its first overseas territories. During the Punic Wars with Carthage between 264 B.C. to 146 B.C., Rome spread over multiple Mediterranean islands and onto the east coast of modern-day Spain. Yet instead of extending its republic into these territories or forming alliances, Rome designated these new territories as provinces and appointed Roman governors to oversee them.

Taking this new territory wasn’t something Rome had initially intended to do. “The First Punic War is something that they kind of stumble into, but they’re happy to take territory as a result of it,” Watts says.

After Rome pushed Carthage out of Sicily in the first war, the Italian island became Rome’s first foreign province. During the Second Punic War, Rome found itself on the defense as the Carthagian general Hannibal and his elephants marched over the Alps and south into Italy. Again, Rome defeated Carthage and conquered some of its territory, this time in Spain.

Yet by the time it entered the Third Punic War, “Rome has definitely decided that it is just going to take territory,” he says. “And that’s very different from what they were doing even in the third century.”

This time, Rome destroyed the capital city of Carthage in modern-day Tunisia and enslaved the city’s inhabitants. It also conquered all of Carthage’s territory in North Africa and made it a Roman province. Rome was now the major hegemonic power in the Mediterranean region. Over the next century, it cemented its status by conquering coastal territory in the modern-day countries of Greece, Turkey, Egypt and others until it completely surrounded the Mediterranean Sea.

After that, Rome used its impressively large army to extend outward in various bursts, sometimes just taking advantage of neighboring states and kingdoms as they fell. In the 60s B.C.E., Rome extended into the Middle East and captured Jerusalem. These eastern territories had old and complex political systems that Rome largely left in place.

The next decade, General Julius Caesar led Roman soldiers into northwest Europe, “basically because Caesar decided he wanted to do it, and he had troops that were capable of doing it,” Watts says. “It’s the way Caesar kind of made his career.” The Roman approach to these western territories was slightly different, because they didn’t have old, complex political systems. When Rome took over, it introduced some Roman systems, while still trying to keep power in the hands of local leaders to ensure a smooth transition.

In addition to pushing Rome’s reach across Europe, Caesar also heralded the end of the republic and the beginning of the Roman Empire. After unconstitutionally declaring himself dictator for life, senators murdered him in 44 B.C. The republic fell for good when his great-nephew, Augustus Caesar, declared himself emperor in 27 B.C. Now, the sprawling state of Rome was officially the Roman Empire.

The empire reached its peak in 117 A.C. when it fortified its borders and reached all the way into England. But after that, it stopped expanding, because leaders didn’t think it was worth the time and energy. The bare-bones imperial structure that let provinces govern themselves made the whole thing manageable until 212, when the Roman Empire extended citizenship to all free people (free women were still citizens even though they had fewer rights than men).

But the extension of imperial bureaucracy made the empire much harder to manage; and this was one of the reasons that the empire began to divide itself. The year 395 was the last time that the whole empire was united under one emperor. After that, the western half split off and collapsed within a century. In the east, the Roman Empire—also known as the Byzantine Empire—continued on for over a millennium.