Well I've always enjoyed the lore that the TES series creates and follows. So the other day I read that after the Empire had signed the White Gold Concordat the people of Hammerfell continued to fight the Thalmor. And after a bit of fighting and such they overthrew the Thalmor in their land and signed a treaty with them, after the Empire had dropped all connections with the people of Hammerfell. It also says that the Redguards questioned the Empires strength as Emperor Titus Mede II was able to destroy the Thalmor army in Cyrodil, so why couldn't they keep fighting and defeat the Altmer?
What happened was, that once the Thalmor lost the element of surprise and lost their momentum, they puffed up their chest to make themselves look bigger and more intimidating, and bluffed. Emperor Titus Mede II was too much of an idiotic fop to see through the smokescreen, so he agreed to the WGC. The Redguards however, saw through the bluff, and called it. The Redguards then proved the Empire wrong, by beating the Thalmor out of their lands. Now, the Redguards hate the Empire, because the Empire tried to sell them out to the Thalmor, in order to save it's own ass.
thats why I help the empire,they will build strength and then catch the Thalmor off guard
then we rip up the white gold concordant and get back to buisness
How is the Empire going to catch the Thalmor off guard, when the Thalmor has planted spies and informants all throughout the Empire, and the Empire has let them do so freely. The Emperor likely couldn't even take a [censored] without the Thalmor knowing it. Any and all battle plans the Empire draws up, will be known and thwarted by the Thalmor. The Thalmor will know when, where and how the Empire intends to strike. The Thalmor could intercept an order for a troop movement, then change it to where the Imperial Troops end up marching right into an ambush.
The only time the Thalmor is taken by surprise, is by immediate, spontaneous action. It doesn't give their spies enough time to gather and relay information.