Just wanted to ask some question about the Impearial Legion and the Empire.

So i have heard that they are not the strongest anymore?

Oh .. and what excatly is happening In Cyrodiil?

They really are reflecting the real life Roman Empire when it started to die. Bad rulers, being defeared by foreign powers and rebellion.
Thats the western roman empire anyhow. The eastern roman empire lived on into the 1400s.

This.

At this specific point in the 4th Era, the Empire is becoming plagued by internal problems caused by the White-Gold Concordat.

• Disallowing the worship of Talos has weakened the memory of Tiber Septim, he who founded the Empire, leaving them without an idol and disolving the normal way of life and worship in the Empire.
  
• Without Tiber or his godly aspect Talos, the outward image of the Empire's strength becomes weaker. This in turn, begins to destroy the cultural identity of the Empire and its people.
  
• The Aldmeri interference in to the religious preferences of the Empire have weakened its relations with its bordering territories, especially the territory of Skyrim, inciting civil war amongst the "states" of the Empire. This creates further internal issues, as the people of the Empire start to question the solidarity of Imperial rule, and the autocratic nature of the Emporer who continues to sell the Empire and its culture to the Aldmeri. (The Caeser effect!)
  
• No side isn't a pawn: The Aldmeri are playing chess with the human races of Tamriel. The Emporer himself is the biggest pawn, believing himself in control as the Thalmor constantly guide his hand and conspire against the whole of the Empire. The Stormcloak rebellion in Skyrim..? Just another pawn in the game. The Dominion has succesfully legislated itself in to a position to watch mankind tear itself apart by artificially creating a civil war. (See bullet above)

I could go on and on with the bullet points...
TL;DR..? The Empire is collapsing in upon itself because of the Dominion. The events taking place in Tamriel are not random. They are elements of a master plan on behalf of the Dominion, to exterminate the lesser races of Nirn and return Tamriel to the time of Aldmeris.

Eh, everyone else really summed it up. So far it is going like the Roman Empire, as others have said.

Oh Martin, why did you have to go to another dimension; you should of at least had one bastard child.