|This article, Galactic Empire, contains spoilers. Be forewarned, plot and/or ending details follow.
The First Galactic Empire, more commonly known simply as the Galactic Empire or Empire, or sometimes called the First Empire, Galactic Empire of Humanity, Empire of Humanity, Human Empire, New Empire of Humanity, Imperium or Old Empire, is a group of planets settled by humans across the whole galaxy in Isaac Asimov's Robot series, Empire series, and Foundation series of novels. It is clearly based on the Roman Empire.
The Galactic Empire consisted of twenty-five million inhabited worlds, all colonized by Settlers from Earth, as opposed to Spacers. The Galactic Empire was ruled by a single man, the Galactic Emperor. The Empire's capital was Trantor, a planet covered entirely by a giant city, located near the center of the Galaxy. It was both the center of power in the Galaxy and also the administrative head.
In the year 11,586 AD, or 1 GE, Emperor Frankenn the First was coronated, which marked the transition from the Trantorian Empire controlling half of the galaxy to a virtual Galactic Empire which slowly began to entirely dominate the Milky Way. By the year 827 G.E. (Galactic Era), this Empire comprised millions of inhabited worlds with five hundred quadrillion residents, and was made possible by the ability of humans to travel through hyperspace. The space navy of the Empire was called the "Imperial Navy".  During these early centuries, whole sections of the Galaxy would now and then refuse to accept the end of their local indepence, and there were vicissitudes that went with the occasional rebellions, the dynastic wars, and some serious periods of breakdown, but the dominating kingdom that was this Empire survived all this and was officialised as an accepted Galactic Empire around 2,000 GE.
The Golden Age
In the 12,000s GE, a mathematician called Hari Seldon, by the means of psychohistory, predicted the fall of the Empire. He predicted that the empire would collapse within 300 years, leading to a 30,000 year period of anarchy before a Second Galactic Empire would be established. To influence events so that the interregnum period will be only 1,000 years and not 30,000, he created a small haven of technology in a corner of the galaxy (on the planet Terminus) called the First Foundation, whose job it would be to preserve knowledge from the collapse, thus reducing the time required for the next Empire to rebuild.
Fall of the Empire
Two hundred years later, even more of the Periphery had degenerated into barbarian kingdoms, and the Empire only controlled the inner third of the galaxy - but this inner third was always its core power-base, controlling three-quarters of the entire galaxy's wealth and population. As a result, many in the inner provinces still openly derided the suggestion that the Empire was in a "decline" at all. The Empire entered into a final renaissance of power under the last strong Emperor, Cleon II, if only because two centuries of civil wars over the throne had finally ended in exhaustion. Cleon II's reign at least managed to stabilize the Empire from further decline for a generation.
This relative quiet allowed word of the First Foundation's expanding sphere of influence in the Periphery to come to the attention of Imperial officials. Ultimately the Empire sent an expeditionary fleet led by general Bel Riose to conquer the Foundation, in a short-lived period of renewed Imperial expansion not seen in centuries. Riose repeatedly defeated the Foundation through brilliant tactical and strategic planning, but before he could complete his conquest, he was recalled by the Emperor, tried for treason, and executed. As Seldon predicted, a strong emperor would always fear that general might become strong enough to dethrone him - the historical pressures on the Empire were simply too great for it to escape decline.
Following the defeat of Bel Riose and the subsequent death of Cleon II, the Galactic Empire falls into a civil war. After this civil war an even more rapid phase of decline begins, with weak emperors losing vast amounts of territory and strong ones barely holding onto what they have. Three-hundred years after the Foundation's creation, the Galactic Empire's capital world, the city-planet Trantor, is sacked.
The Empire now bases itself on Neotrantor, which is almost mockingly named so; it was an agricultural colony that used to feed Trantor. The Galactic Empire, which used to touch every planet in the Milky Way galaxy, is reduced to twenty agricultural planets. It is dwarfed by most barbarian kingdoms.
The greatest conqueror the Galaxy has ever seen, The Mule, defeater of the First Foundation, later conquers the pathetic remains of the Galactic Empire as well in the 4th century FE.
The Periphery refers to the outer rims of the Galactic Empire, including planets such as Anacreon and Santanni. Imperial control is weakest in the periphery, as planet governors there frequently possess their own fleets. Santanni revolts. About fifty years after the First Foundation is established on Terminus, the emperor grants the periphery autonomy. This effectively removes them completely from Imperial control, making the dissolution of the Empire far more apparent than it had been before.
In the years preceding the fall of Trantor, the periphery became any area outside the empire; as this area became larger the empire became less and less great. As the empire decreased in size and power, the stagnation that would ultimately cause the collapse of the empire increased.
- Asimov created the Galactic Empire in the early 1940s, based upon the Roman Empire, as a proposal to John W. Campbell after reading Edward Gibbon's The History of the Decline and Fall of the Roman Empire when he was working at the Philadelphia Navy Yard with Robert Heinlein. The concept evolved through short stories and novellas in Astounding Science Fiction magazine during the 1940s, culminating in the publication of the Foundation stories as a trilogy of books in the early 1950s.