From what I have read, the Great War that led up to the White-Gold Concordat between the Almeri Dominion and Empire was a war that the Thalmor were winning. The Empire vastly underestimated the Thalmor and had they not signed the concordat, they would have been defeated. This makes me wonder if the Thalmor had the power and strength, why didn't they just conquer the Empire while it was weak and reign total control over it as they saw fit? Wouldn't this give the Empire a position to eventually strengthen and revolt for independence free of Thalmor pressure and influence?

My other question was, what made the Thalmor and Summerset Isles so easy to conquer under Tibet Septim's reign?

1. They were exhausted too. Other than that we don't have much inight into the specifics of their gameplan.

2. The Isles were conquered by the giant robot god Numidium. The Thalmor of that time was not the current Thalmor, and I don't think they were really even in charge of Summerset at that time - they were a body that governed Valenwood as a province of the Aldmeri Dominion. We hear of Zurin Arctus meeting with the King of Alinor in the PGE1, so it's pretty clear that the Altmer monarchy was still in control back home until the third Thalmor assassinated them all and purged the resistance in the fourth era.

Signing the White-Gold Concordat didn't necessarily save the Empire from defeat. Instead, it was a mark of defeat. The Dominion had originally issued a list of concessions from the Empire prior to the war. These were things they wanted and - if they didn't get - they were prepared to go to war over. The Empire initially refused: and the subsequent defeats of the Imperial forces gave the Dominion more than they had ever asked for. This was reversed following the Battle of the Red Ring, where Imperial troops turned back the Dominion from its territorial holdings in Cyrodiil. Sufficiently exhausted, the Empire gave into most of the Dominion's former demands, which now seemed a small price to pay compared to what had been lost in the early months of the war. The Dominion, defeated on the battlefield, was none-the-less successful in achieving their aims in the conflict. They had no reason to continue the war at this moment, and peace - as it would for the Empire - would let them recover and plan their next move.

Well, it wasn't easy. The Aldmeri Dominion of the 2nd Era effectively held Tiber Septim's Empire in a stalemate for many years. While the Legion had the advantage on land, often defeating the Dominion's armies, the Dominion held the advantage at sea, and utterly decimated Imperial fleets on several occasions; most notably at Black Rocks. The jungles of Valenwood were also notoriously difficult to penetrate.

What tipped the scales so drastically was the acquisition ofhttp://aurbis.c0da.es/c0da/s/hargrove/s13.jpgby the Empire from the God-Kings of Morrowind.http://th08.deviantart.net/fs71/PRE/i/2011/247/6/4/numidium_by_thobewill-d48wcxt.pngwas a powerful Dwemer automaton similar to the Dwemer Centurions you encounter in Skyrim, but vastly larger and more powerful because it had been instilled with quasi-divinity. In essence, it was a walking god. When the Numidium was activated at the Halls of Colossus in Elsweyr, the flow of time itself was shattered and the Numidium conquered the Dominion in the space of an hour, with Imperial forces following in its wake.

There is a reason why we know Tiber Septim as http://en.wikipedia.org/wiki/Talos.