So apparently they were not an invasion force sent from Akaviri to annex the lands of Tamriel.
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Pocket Guide to the Empire (3rd editionIn the 2703rd year of the First Era, Tamriel first faced an organized armada of Tsaesci, the so-called "Akaviri Snakemen", and met the challenge with a resounding victory in the Pale Pass of Skyrim. The Emperor Reman was so impressed by the exotic weaponry and battle prowess of his defeated foe that he and his heirs allowed them to rise from prisoners to advisors of the second Empire of Tamriel. Eventually, they became, on the death of Emperor Reman III in the year 2920, the Potentates of Tamriel, and the defeated Akaviri ruled the land for over 400 years.
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In TES IV Oblivion, the Champion of Cyrodiil was sent on a quest by the Countess of Bruma to find Akaviri trinkets in the Pale Pass in the Jerall Mountains (Cyrodiil/Skyrim border). The Countess had also given the hero a translation of a diary of an Akaviri messenger (http://www.uesp.net/wiki/Oblivion:Akaviri_Diary_Translation). Nowhere in the text, does the messenger give any hint about hunting dragons or finding a dragonborn other than expressing contempt towards Emperor Reman.
Later on in the quest, the Champion finds the secret Akaviri command post in the Pale Pass and discovers that a great landslide had covered the fort thereby trapping the Akaviri commander and leaving the Akaviri columns leaderless against the Reman onslaught. The Champion also happens upon the ghost of the Akaviri Commander, Mishaxhi, who still believes himself in charge of the expedition. If the Champion informed Mishaxhi that he/she/it was a soldier with the Army of Reman, Mishaxhi attacks immediately, calling the Champion a "Reman dog," stating to the effect that he'd never surrender to the dogs of Reman.
Had not the landslide claimed the lives of Mishaxhi and his men, the Battle of Pale Pass could've turned out very different. Either the Akaviri would've defeated Reman and conquered Tamriel or the Akavir would've lost any way but due to the diehard nature of Mishaxhi, he would've fought to the death. Without command leadership and the loss of manpower because of the landslide, the rest of the Akaviri, isolated in an alpine wilderness and surrounded by a sea of enemies, had no choice but to surrender to Reman and his forces.
I don't believe the Akavir had no choice but to serve Emperor Reman or die. As it stands, I can't accept the notion that the proto-Blades were historical dragon hunters and not Akaviri invaders and conquerors. I find the Skyrim books about the Blades/Dragonguard very sketchy and especially when conversing with Esbern and Delphine. I don't like those old coots.