Board Thread:Lore Discussion/@comment-81.107.68.230-20130302224601/@comment-5227390-20130302234732

Actually, their issue with the Blades pre-dates the Great War, as evident in the Thalmor's actions against the Blades in Summerset and Valenwood prior to the Great War's start.

The Blades were operating in secret methodical ways in the realms of the Mer as well as other provinces. Their goal was to be a proponent of the Empire's expansion and control, also in order to secure the safety of the Emperors. Once the Dragonborn Emperors died out with Martin being the last, they were still abroad in the different provinces, working toward their own goals, though still considered part of the inner workings of the Empire.

The Thalmor didn't like this hidden hand of the Empire sifting amongst their own borders... so as they moved against the Empire, the Thalmor by extension needed to erradicate the Blades as they were a much more dangerous and sneaky faction than those of the Empire's armies.

They hunted down and decapitated all of the Blades' agents within Valenwood and Summerset. They then offered a somewhat fake ultimatem to the Empire to condemn and disband the blades. The Emperor refused only to discover the Thalmor had already made up their minds to destroy the Blades, and move against the Empire. This led to the Great War.