The First Legion: (1E 242 - 1E 2331) Otherwise known as The Alessian Empire, was forged from a slave rebellion of Nedic peoples against their Ayleid masters. There were many rises and falls in the span of the Alessian Empire's control, but it still lasted for over 2,000 years. Nevertheless, it was with Queen Alessia, the first of its rulers, that made a pact with the god Akatosh that forged a bond between gods and men in order to protect Nirn from the denizens of Oblivion.
The Second Legion: (1E 2703 - 1E 2920) Otherwise known as The Reman Empire, or Cyrodiilic Empire, was a short successor to the Alessian Empire. Though short-lived, it was nevertheless considered a heavenly ordained successor to the Alessian Empire. It was named for Reman Cyrodiil, who defeated an invading Akaviri force, though he never held the title of Emperor himself, only his descendants.
The Akaviri Potentate: (1E 2920/2E 1 - 2E 430) Also known as the Tsaesci Potentate, was a period of the Second Legion's history in which Akaviri Potentates ruled the Empire. In the events of the conflict between Reman Cyrodiil and the Akaviri Invasion, many of the Akaviri regarded Reman with praise, as they saw he was Dragonborn. During the time of the Potentate's direct rule, there were some benefits, but the Potentates employed totalitarian practices and made enemies of the governing bodies of the subject states within the control of the Empire of the time. Eventually, though Vampiric and immortal, a revolt would tear down the Potentate's control. The Potentate's rule would ultimately end when the Morag Tong were employed in assassinating the then-current Potentate and all his heirs.
The Interregnum: (2E 430 - 2E 854) A chaotic period of time in which misrule and constant conflict arose, after the fall of the Reman Empire and before the beginning of the Third Empire. It began with the Morag Tong assassins killing the Potentate, following the orders of their Night Mother. This event is accredited as the birth of the Dark Brotherhood. Multiple members of the former subject nations of the Empire warred with each other over a thirst to rebuild an empire out of their visions. This era ultimately ends with the conquests of Tiber Septim.
The Third Legion: (2E 854/3E 1 - 3E 433) Also known as The Septim Empire, or Tamrielic Empire, was forged through several momentous events at the hands of the conqueror, Hjalti Early-Beard. Hjalti was called by the Greybeards and named Dragonborn. It was said he was destined to rule all of Tamriel. He took on the name Talos Stormcrown and began his efforts to reunite the Empire of old and expand to control all of Tamriel. When he formed his own, official, empire, he once again changed his name and took on the name Tiber Septim. The Septim Dynasty had numerous history-making rulers, but the biggest event of its lifetime was its ending, crumbling beneath the might of the Daedric Prince Mehrunes Dagon and his Oblivion Crisis.
Oblivion Crisis: (3E 433 - 4E 17) With the death of the last Septim Emperor, Uriel VII, the hunt for an estranged heir was set forth on. Martin Septim, an illegitimate son of Uriel VII, was a Priest of Akatosh in the city of Kvatch. With the onslaught of Daedric forces all across Tamriel, however, bringing him to the Imperial City was a tough task. With the discovery of the depth of the Mythic Dawn's plans, and the machinations of Mankar Camoran, the Daedric Prince Mehrunes Dagon was, himself brought to Nirn. In the end, Martin Septim had to sacrifice his own life to become an avatar of Akatosh to put an end to the Crisis. Though this event occurred within the 3rd Era, and was the primary fight against Dagon, the fight against the Daedra was waged in all provinces of Tamriel by different forces. On top of this, a band of Warlords in Cyrodiil then waged a civil war due to differences over who should take up the throne in the absence of a Septim heir.
The Fourth Legion: (4E 17 - 4E 215) Also known as The Illegitimate Empire, or simply, The Mede Dynasty, is by some argued whether or not it constitutes a new Empire, or simply a dysfunctional ending to the Septim Dynasty's Empire. After a short interregnum known as the Stormcrown Interregnum, Titus Mede I was accepted by the Imperial Elder Council as an acceptable ruler over the Tamrielic Empire. However, the Mede Dynasty was plagued with pitfalls as the Empire destabilized and collapsed beneath their rule. The First Great War, between the Tamrielic Empire and the Aldmeri Dominion brought about a civil war in Skyrim, and severing of ties with Hammerfell. After the Skyrim Civil War, the Second Great War brought about the official collapse of the Tamrielic Empire, whether it be considered the Third or the Fourth.
The Aldmeri Demesne: (4E 215 - 4E 230) A short period, but still a heated period of history. This time period involves the recourse of the Second Great War, and the beginning of the Aldmeri Dominion's control over the realms of Tamriel. Throughout this period, the Aldmeri Dominion turns Cyrodiil into a puppet state and leaves Skyrim in disarray as a subject state. The Dominion then begins to make efforts to send spies and agents out to extend their control over Hammerfell, High Rock, and Morrowind as well. This period comes to an end at the beginning of the Third Great War.
The Fifth Legion: (5E 1 - Present) After the end of the Third Great War, a reformation and rebuilding period occurred for a few years, forming the transition period between the end of the Aldmeri Dominion's control over Imperial territories and the return of the Tamrielic Empire, or in this case, The Corvinus Empire, under new leadership. Hero of many endeavors and wars before the beginning of the 5th Era, Svarog Svanirsson took on the Imperial name Aeneas Corvinus and began a new dynasty of Imperial rulership.