Great War

The Great War (4E 171 - 4E 175) was the name given to the War between the Thalmor of the Second Aldmeri Dominion and the Empire of Tamriel during the Fourth Era. The Thalmor call it, quite forebodingly, "The First War with the Empire."

Thalmor and the Medes
After establishing their power in Summerset Isle, the Thalmor engineered a coup in Valenwood and eventually annexed it, and also drew Elsweyr into their sphere of influence. According to Esbern, the Great War began on the 30th of Frostfall in 4E 171, when Emperor Titus Mede II rejected an Aldmeri ultimatum, and the Dominion in turn launched large-scale attacks on Cyrodiil and Hammerfell. The Dominion came close to victory, managing to take the Imperial City and drive the Legion and Redguard forces out of most of southern Hammerfell. But the Empire rallied, taking back the Imperial City and destroying the Thalmor army occupying Cyrodiil.

The White-Gold Concordat
Soon after this, the White-Gold Concordat was signed, officially ending the war. The result of this treaty ended in two notable events. First, large portions of Southern Hammerfell were ceded to the Aldmeri Dominion. The Redguards, outraged at the Empire paying for the treaty with their hard-won lands, began their own separate war with the Aldmeri. This resulted in the Empire renouncing Hammerfell as an Imperial province and, in effect, abandoning the Redguards. Through the resilience and might that has become expected from them, the Redguards fought back and defeated the Aldmeri, but relations between Hammerfell and Cyrodil have become bitter as a result. Secondly, the worship of Talos was outlawed, and the Nine Divines became the Eight Divines once more. Many Nords saw this as an affront, particularly Ulfric Stormcloak; some continued to worship Talos in secret, and the anger over the White-Gold Concordat is one of the driving forces behind the current civil war in Skyrim. The White-Gold Concordat was signed in 4E 175, consequently ending the Great War. The war lasted for 4 years.

Aftermath
In all accounts, and with all things concluded, the result of the Great War is simply summarized: the Aldmeri Dominion is now the dominant force in Tamriel.

Ominously, Thalmor correspondences found in Skyrim have revealed a different name to describe the Great War: "The First War with the Empire." This suggests that not only do the Thalmor intend a second Great War, but have already begun to plan for how it can be executed.

Dark Brotherhood
The Dark Brotherhood of Cyrodiil was accidentally eradicated during the Great War. The Lucky Old Lady statue masking the Night Mother's remains was also destroyed, and the Mother's Listener, Alisanne Dupre and many other members were murdered by Thalmor Wizards.

Trivia

 * While the creators of Skyrim/The Elder Scrolls/High Fantasy do take inspiration from real history, one should not compare them literally. If comparing the current Empire(Elder Scrolls/Skyrim) to any empire in real history, it would show the most similarities with the Roman Empire. It was founded by a very capable leader and remained strong for generations due to strong leadership, but then slowly dwindled under bad leadership and loss of its identity over time. Today(Skyrim), as the Empire crumbles, new, stronger, more vibrant and uniform states/kingdoms emerge such as Hammerfell and, should one choose to support the Stormcloaks, Skyrim. (After the fall of the Roman Empire you had kingdoms like France, England, Germany HRE emerge, while some remnants of the Empire lived on).