The Roman empire is the longest standing empire in history, over 1000 years which is quite an accomplishment. In fact, Pontius Pilate under the command of Julius Cesar's HQ in Rome was the one who allowed the crucifixion of Jesus, his subsequent death and resurrection 3 days later.
Why the Romans were a very large empire, they didn't conqueror their enemies quickly or overnight like the speedy Monguls before them. The Romans were instead very much methodical, generation after generation. At their core the Romans were engineers and builders, they were also more cruel than anyone who came before or after them.
Julius Cesar invented "Total War" when he invaded the Gauls, present day Germany.
Total War is the idea you pull out all the stops, anything goes, and that meant anything. What Commodus did to Maximus's wife and child in the Oscar hit Gladiator is just a drop in the ocean. Eye for an eye is simple, Total War is more strategic and attempts to exploit any weakness against your enemy, his family, his home, his county, and his dignity as an example to others to stay in line. The idea is you make war more painful for your enemy so they typically give up before any spears are thrown and thus become subjugated and members of the Roman Empire.
Since the Romans were always outnumbered, they had to scrappy. Julius Cesar was the most refined of them all as he conquered all present day Germany & France occupied by the mighty Gauls. The Gauls were in fact, Rome's mortal enemies having sacked Rome BC 410 and again in years past.
So where does all this lead? I live in America and we do not engage in total war even though we could. We have the best weapons and a huge defense budget rivaled by none. But America has learned a better way. America believes in protecting freedom of all humans around the world, regardless of their social status or creed, we do not aim to kill innocents and civilians. We protect the downtrodden and let their people judge them such as Saddam Hussein.
In closing, since the Romans were always outnumbered that had to win in other ways, and those ways were very cruel, sometimes called worse than barbaric. They did however quite literally pave the way for future generations and hope that history would not repeat itself.
To learn more and a good podcast in the car or office, Dan Carlin and Elon Musk discuss an Engineers War and the Romans way of doing things or Google EP17 Engineering Victory with Elon.