Board Thread:Lore Discussion/@comment-24137988-20140425080241/@comment-75.75.80.173-20140425221935

The Empire seems to be a lot less stable with Mede Dynasty, the loss of several provinces, but likely political infighting as well.

With people like Maven Black-Briar in power in the Empire for some time, corruption could have weaked the Legion greatly  (they seem to have downgraded quite a bit since the Oblivion Crisis).

The Empire probably was at a most vulnerable and opportune time for the Thalmor to attack, though the resistance may have been slightly underestimated.