The Thalmor is the ruling faction of the Aldmeri Dominion, responsible for aggressive expansionist policies which ultimately brought it to war with the Tamrielic Empire during the Fourth Era. The Thalmor were successful in severely crippling the Empire during the Great War which ended in an uneasy truce between the two powers through the signing of the White-Gold Concordat.
 Rise to Power
Though the Thalmor had its roots in Valenwood (homeland of the Bosmer) as far back as the Second Era before the nation officially became a province of the Empire during the reign of Tiber Septim himself, it was in the Summerset Isle where the faction succeeded in rising to prominence during the Oblivion Crisis of the Third Era.
While much has been made of the Champion of Cyrodiil's efforts to undo the Oblivion Crisis within the Empire itself, the Thalmor claim it was through their efforts that the Summerset Isle were able to succesfully fend off the threat on its own. Ultimately the Thalmor were able to seize control of the Isle in 4E 22, renaming the Altmer nation "Alinor".
The Thalmor finally succeeded in gaining control of Valenwood's government in 4E 29 thanks to collaborative efforts which were seeded long before the Oblivion Crisis ever began, and the Aldmeri Dominion was officially formed with the Thalmor as its ruling body.
 The Great War
The Thalmor were so successful in gaining territory in the early Fourth Era (including Elsweyr which was dissolved into the kingdoms Anequina and Pelletine in 4E 115) that in 4E 171 it issued an ultimatum to the Empire itself made of staggering demands and tributes including control of Hammerfell, the disbanding of the Blades, and a ban on the Worship of Talos as one of the Divines. These demands ultimately sparked the Great War with the Empire in 4E 171.
The Great War finally ended in 4E 175 with the Empire largely exhausted and crippled. Emperor Titus Mede II negotiated for peace with the Aldmeri Dominion, signing the White-Gold Concordat which in fact realized many of the Thalmor's original demands including the banning of the worship of Talos. This compromise would anger many of the Empire's remaining allies, including Skyrim and Hammerfell.
The Thalmor's agents continue to operate in the Empire's remaining provinces, enforcing the ban on Talos worship. Its dignitaries are welcomed as ambassadors in the Empire, though naturally many have agendas that serve the Thalmor's interests.
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