Wars aren't just won with battles. Wars are fought on many levels. Look at the Dominion's food situation. You can't grow food in Valenwood with the Green Pact happening. Half of Elsweyr is desert. That leaves their only bread basket possibility on the main land as the other half of Elswyer which is jungle. The main food supplies probably from Summerset, an island days or weeks away from their holdings on the main land. You have to transport by sea, and then by land. That's a major supply problem right there.
Cyrodiil is the bread basket of Tamriel. Their farms are in the perfect location for massive operations. If they get busy with their farms, that will be a massive advantage over the Dominion.
In reality, the Aldmeri aren't in a position for war either. They had entire armies destroyed in the war. They're tiered, and need troops. Men recover losses faster than elves. They're just about ready. Another five years, and the Empire will have 2 generations of troops to pull from. There was only one generation between WW1 and WW2. The Empire will go for a hammer stroke.
The Dominion already beat an Empire of united human kingdoms. The Imperial City fell and the Empire lost more than half of its Legions in the two battles placed there. Cyrodiil was dependent on reinforcements from Skyrim just to stave the Aldmeri forces off for as long as they did, and they had to completely abandon the 2nd campaign the Dominion lead against them, in Hammerfell. Before the war broke out, Cyrodiil is described as being largely peaceful.
Now, in the aftermath of the war, we have at least two cities that fell. Bravil erupted in violent gangwars, while Cheydinhal fell to unspecified forces. The innkeeper in Falkreath also mentions something vague about "fighting in Cyrodiil". Then subtract the reinforcement Cyrodiil received from Skyrim and Hammerfell from the equation.
Cyrodiil, being stable(good for food production), with 3 allies, barely avoided a complete loss against the Dominion because the Dominion made a mistake, and managed to get it down to an appeasement policy that is tearing the Empire apart and furthering the Thalmor influences in their lands. How is a much less stable Cyrodiil(less good for food production), with fewer allies, going to win this?
The Empire lost 50%+ of their forces the last time. The Dominion lost their "main invasion army", as it was described in The Great War(book), for the Cyrodiil campaign. They still had a working army that occupied half of Hammerfell for another 5 years before they left by treaty. Now you have to reduce the Empire's total forces by a great deal, since they can no longer recruit from Skyrim(The Empire's Strongarm according to the PGE 3rd Edition for the province Skyrim) or Hammerfell. Where do you get the manpower for the Empire to go for a hammer stroke here, since it took the Brass God(Numundium) to invade Summerset the one and only time anyone have ever succeeded at that. Or, an easier question, where do you get the manpower for them to even push out the Dominion from Valenwood and Elseweyr?
How does this make Valenwood, Elseweyr and, most importantly, Summerset Isles easier to invade?
And, as I mentioned, the only army that was destroyed was the main army for the Cyrodiil campaign. They still have their Hammerfell campaign army, since it was never destroyed. And if it took Cyrodiil with 3 allies more than half of their troops to deal with one army of the two...
Humans breed faster than Mer, but I'll need a source if you're going to claim that they breed so fast that Cyrodiil can take up the complete loss of two whole nations as recruitment polls for soldiers.
AD doesn't have the manpower, Empire outnumbers them greatly already. 20 years from the Empire will be prepared and willing to fight again, with, or without Skyrim. Many Nords would also leave Skyrim for Cyrodiil, many Nords care more for the Empire than for Skyrim alone. The ones with actual intelligence would help the Empire to save Skyrim, as there's nothing except Akaviri or Divine intervention that would stop the AD from throwing down mankind from power once they have destroyed the Empire.
When I asked for a source, I wanted something from the wiki or uesp. Something with a Bethesda stamp. But if we play by your rules...
Source: The Aldmeri are over exerteted. They are bluffing. They can't back up their talk with force. They are mouthy spoiled brats. They're carrying a bruised ego. Old men still holding on to the bloody handkerchief from the punch received when they were children. Ha! Some beacon of civility they are! They're so mature! They live thousands of years, but they never grow up. That's not opinion that's FACT!
No, not formal. Formal is waiting at least 1 season, you attacked the same fecking day.
Your point is still invalid, the legions were scrambled as there were more in the provinces (Including Summerset Isle, so be thankful). You had 1 province to protect, the Empire was busy on every single one.