I am [censored]led annoyed by wich side I will join!

Tell me if that proceeds:

1 - The Stormcloacks started the rebelion. The Empire was doing it's very own business before the rebellion.

2 - The concordatta is just a way the Empire found to "hold on" untill get forces to attack the thalmor

3 - The Imperials are not allies of the Thalmor.

4 - Ulfric has no chance against the thalmor if he split skyrim from the Empire.

5 - Ulfric is not smart enough to deal with the thalmor threat.

6 - Why the hell Tullius told the soldier to cut off my head!??!

7 - Who Am I?

Depends who you ask. The Stormcloaks might consider the rebellion started as soon as the White Gold Concordat was signed and the better part of Nordic religious tradition was made illegal.

Maybe. Whatever time it gives the empire, it also gives the Thalmor.

No, but given the Thalmor running around enforcing their own whims in Skyrim, it doesn't appear that the Empire is a worthy opponent of the Thalmor either.

Says who?

If you're talking about the start of the game, that isn't Tullius...