Roman Empire
The Roman Empire (Imperium Romanum) was a civilization which was based out of the Italian City of Rome. It was notable for unifying half of Europe and the Mediterranean under their rule and being one of the most advanced civilizations of Classical Antiquity, with only the Chinese Han Dynasty, Indian Satavahana Empire, and the Parthian Empire (With the Persian Empire succeeding it) rivaling it....
Roman History 101
The Early days
According to the legends, Rome originated from a group of butthurt Trojans who escaped the Trojan Wars under the leadership of Prince Aeneas, we roughly know where Troy is so that means they had to sail all the way from Turkey, not too unlikely however given that the Greeks had Colony all over the Mediterranean. Aeneas' lineage would eventually give birth to Romulus and Remus who formally founded the city of Rome in 753 BCE... by enticing Sabine women to come to their little city, and while everyone was drunk, Romulus would then signal to rape (meaning abduct) every Sabine woman in sight, which naturally pissed off the Sabine men. Luckily however, Stockholm Syndrome prevented war from breaking out and Rome got to live on. Somewhere down the line the Roman kingdom is no longer ruled by a Roman, but by an Etruscan, whose son rapes (meaning rapes) a girl named Lucretia. The Romans decide that kings are bad, and reforms as a Republic.
The Punic Wars
Meanwhile, in what is now Tunisia there was another power on the rise, a city called Carthage. Eventually there began to be some friction between the two groups as Carthage moved into Sicily and Sardinia, compromising Roman trade in the Mediterranean. This led to three wars called the Punic Wars where Rome and Carthage clashed.
The first Punic War (264 BCE to 241 BCE) involved the Romans fighting the Carthaginians over Sicily and Sardinia. The Romans begin to take to the sea for the first time. Rome got its ass handed to it at first, then reverse-engineered a Carthaginian ship and started pushing back. Instead of letting the war draw out, Carthage sued for peace, and gave Rome very generous terms, which they broke constantly over the next decade or two. This lead to resentment in Carthage, and set the stage for part two.
Eventually shit happened involving a Greek colony and war sparks again. A guy named Hannibal, whose father commanded the Carthaginian forces last time, swears revenge on the Romans. He invades Spain, brings elephants over the Alps and into Italy (at heavy cost), and generally starts wrecking shit. The only thing holding him back is Fabius "The Delayer" Maximus, who used hit-and-run tactics to keep the army away from Rome, intending to starve him out rather than risk a direct fight.. Meanwhile, Roman general Scipio Africanus leads his army to Carthage, Hannibal is called back to defend his home, and gets defeated. Carthage then becomes a dependent "ally" to the Republic, basically being sucked dry of its resources. Eventually around 149 BC Rome saw Carthage as too much of an eyesore and declared war for the third time. A few years later Carthage itself was destroyed, the earth around the city was salted, and the surviving population was sold into slavery.
The Height of the Roman Republic
With Carthage Dead, there was nothing left that could stop Rome from taking over the Mediterranean world. When Rome wanted to conquer an area they gave the local rulers two choices: Surrender and pay taxes to the Glory of Rome, or resist and be purged. Either way, Rome got more territory, more wealth, and more slaves, and expanded more or less uncontested until they got to Gaul. These uncivilized brutes continued to resist until the Romans got sick of their bullshit and sent in Julius Caesar with a few legions to deal with it. He did just that, and became very popular as a result. He uses this popularity to get dictatorship, his friend Brutus decides he isn't that cool any more...and you know the rest.
For next two hundred years the now-Empire is under the Pax Romana. It sees some good emperors, a few bad ones, and a lot of stability, wars of succession notwithstanding.
Fall from Greatness
Rome's fall can be attributed to any number of reasons, and any self-styled amateur historian would be more than happy to explain to you his opinions on the subject, but it's widely agreed that there were a number of contributing forces, including but not limited to: Increased logistical sluggishness of maintaining a sprawling empire, the various Legions becoming more loyal to their generals than the empire and less diciplined, economic and famine crises as Roman farmers were outpriced by foreign imports, Germanic barbarian hordes that wanted "in" to the empire, emperors trading places like it's fucking musical chairs, and lead poisoning from Roman sewage systems. The East splitting off didn't do the Western lands any favors either. Eventually, the barbarians got all the way to Rome, sacked it, and made one of their own Emperor. This is the part where most experts say the twitching corpse is well and truly dead.
Things were a bit better in the wealthier East after the split. The Byzantine Empire, as it's now called though they still identified as Romans, drifted from Rome in culture and aesthetic, and more or less carried on without doing much of anything besides splitting off from the Catholic Church to form the Orthodox, and getting its teeth kicked in during the later Crusades. Eventually conquered by the Ottomans in 1453 with the Fall of Constantinople (now Istanbul), ending the last direct political rule of the Roman world.
Legacy
Even after its fall, the Roman Empire left a mark on western civilization in terms of writing, language, military organization, architecture, legal systems and philosophy. Many major European cities like London or Milan started out as Roman Colonies. In certain fields (in particular medicine, sanitation and plumbing) the Romans were more advanced than their European counterparts up until fairly recently. The end of the middle ages is generally known as "the Renaissance", the rebirth of western civilization which did involve some attempts by the upper class to recreate the better aspects of the Empire. For this reason various subsequent western cultures attempted to try to recreate some of Rome's Grandeur. The more obvious of these was the Holy Roman Empire. The Tsardom of Russia proclaimed itself as The Third Rome. Numerous other European monarchs as well as Ottoman Sultans declared themselves as being Emperors or Kaisers/Tsars. Monarchs drew upon the idea of the authority and splendor of the Emperors, Republics drew on the roman concepts of rule of law, elected governments and civil rights.
Roman Empire Analogs in Fantasy
To save time, let's just say the Roman Empire is one of the most heavily copied Cultures in fiction.
- Imperium of Man - While the Imperium also draws inspirations from other political systems, its aesthetics and government are strongly based around Roman designs.
- Ultramar - Ultramar, Roboute Guilliman's home system, is heavily based around a Greek-Roman hybrid empire.
- Codex Alera - Romans end up in a fantasy world and learn how to train spirit Pokémon. That may sound odd but that's actually the literal origin of how the books were written in fact, the author Jim Butcher, was challenged to write a book using two "lame" ideas, the idea in question were "lost Rome legion" and "Pokémon"
- The Galactic Empire of Star Wars fame, which also evolved from a republic.