The Empire was the government of the planet 892-IV. It was a parallel to Earth's Roman Empire. The Empire was divided into several provinces spread out across the planet's continents. (TOS: "Bread and Circuses")
The Empire was formed in the 3rd century by men who James T. Kirk described as parallels of Julius and Augustus Caesar. At the onset, there were slave rebellions, but they were always suppressed. As the centuries past, slaves were given more rights (pensions, health care) and would no longer rebel.
By the 19th century, wars and other conflicts, had ceased to exist on 892-IV.
In 2261, the SS Beagle suffered meteor damage and sent down a landing party to search for iridium on the planet. The resulting near-contamination of the culture forced the government to respond. Proconsul Claudius Marcus appointed Captain R.M. Merik of the Beagle to the position of First Citizen of the Empire and required the Beagle crew to permanently live on the planet, all the while remaining quiet about their extraterrestrial origins.
In 2268, the USS Enterprise also sent a landing party to the planet, while investigating the whereabouts of the Beagle crew. The landing party originally stayed with a group of Son worshipers, before being caught trying to enter the Empire's capital. The landing party was forced into gladiatorial combat before beaming away. Merik was killed helping them get away. (TOS: "Bread and Circuses")