The War of the Blue Divide was a war between Valenwood and the Summerset Isles, which occurred in 3E 397. The Altmer were victorious in this war. They took control of several islands off the coast of Valenwood which had previously been controlled by the Bosmer. In 4E 29, sympathizers of the Thalmor in Valenwood, the homeland of the Bosmer, overthrew the Imperial administration of Valenwood, and joined Valenwood with the Summerset Isles to form the Third Aldmeri Dominion. However, the Mede Empire and its allies in Valenwood would not bow down easily. The Imperial Legion, aided by the Empire's Bosmer allies, fought back against the coup. It was at this time that the Empire first tasted the true might of the Aldmeri Dominion. The Altmer armies invaded Valenwood, driving out the Imperial Legion and its Bosmer allies, and securing Valenwood for the Dominion. The war left the Empire weakened, paving the way for the Great War.