Only problem is that the Empire is weak by lore standards, and the Dominion sees right through it, and deliberately exploiting this weakness for their own benefit.
Thirty years ago the Empire was weak and unprepared. The Empire has been building up their forces and preparing for the second conflict. The only reason Tullius isn't getting the reinforcements he's requested is Titus II won't send them. Tullius as a General knows there are enough soldiers in Cyrodiil for him to crush the rebellion, he wouldn't be requesting them otherwise.
The Aldmeri Dominion can see the Empire is getting close to being ready, the Imperials had a six year head start while the Aldmeri were fighting in Hammerfell. The Thalmor are trying to distract the Empire with the Stormcloak rebellion, to weaken them and turn their attention to other problems.
For all their boasting, the Thalmor were desperate enough to kidnap a Skaal smith to make them weapons, and later on even buy weapons off the player on Solstheim.
The fact to the matter is that TMII rejected the ultimatum the first round while his own Generals are warning him not to do anything that will ignited a war, and the consequences from his actions cost him dearly as it further weaken his military.
His Generals warned him of Military weakness, Titus II rejecting the ultimatum was good. The Empire would have destroyed itself with Civil Wars everywhere. Not to mention the Thalmor didn't intend to destroy the Empire until they found out it was weaker than they believed. Your argument here is pointless, your whole "I would support the Empire had Titus II agreed to the Ultimatum" you sometimes bring up is hard to believe.
Lets just say Titus II agreed to the Ultimatum. Civil War breaks out in Cyrodiil, High Rock, Skyrim and Hammerfell is attacked. The Aldmeri invade Hammerfell, they overrun the province. The Redguards put up very little resistance, it was actually the Legions that halted their advance. Take away the Legions, they're not stopping the Aldmeri. The Empire is now dealing with uprisings, the Legions are trying to control the people. Even Legions would probably fight other Legions.
Then the Thalmor turn towards the Empire, their spies report the Empire is far weaker than they estimated. Even moreso being divided, and infighting.
So I ask you Raijin when you bring up this argument, why the fl*ff would the Aldmeri suddenly leave the Empire alone? They going to do a complete change of character? Go against everything we know about them?
"The Empire is weaker than we thought, they're falling apart with infighting... Well we'll just leave them be, cause we're nice." - Thalmor.