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". 
The Great WarEdit
In 4E 168, Titus Mede II ascended to the throne and inherited a weakened empire. The glory days of the Septims were a distant memory. Valenwood and Elsweyr were gone, ceded to the Thalmor. Black Marsh had been lost to Imperial rule since the aftermath of the Oblivion Crisis. Morrowind had never recovered fully from the eruption of Red Mountain. Hammerfell was plagued by infighting between Crowns and Forebears. Only High Rock, Cyrodiil and Skyrim remained prosperous and peaceful. Emperor Titus Mede II had only a few short years to consolidate his rule before his leadership was put to the ultimate test.
On the 30th of Frostfall, 4E 171, the Aldmeri Dominion sent an ambassador to the Imperial City with a gift in a covered cart and an ultimatum for the new Emperor. The long list of demands included staggering tributes, disbandment of the Blades, outlawing the worship of Talos, and ceding large sections of Hammerfell to the Dominion. Despite the warnings of his generals of the Empire's military weakness, Emperor Titus Mede II rejected the ultimatum. The Thalmor ambassador upended the cart, spilling over a hundred heads on the floor -- every Blades agent in Summerset and Valenwood had been executed. So began the Great War, which would consume the Empire and the Aldmeri Dominion for the next five years. Within days, Aldmeri armies invaded Hammerfell and Cyrodiil simultaneously. A strong force commanded by the Thalmor general Lord Naarifin attacked Cyrodiil from the south, marching out of hidden camps in northern Elsweyr and flanking the Imperial defenses along the Valenwood border. Leyawiin soon fell to the invaders, while Bravil was cut off and besieged.
At the same time, an Aldmeri army under Lady Arannelya crossed into western Cyrodiil from Valenwood, bypassing Anvil and Kvatch and crossing into Hammerfell. Smaller Aldmeri forces landed along the southern coastline of Hammerfell. The disunited Redguard forces offered only scattered resistance to the invaders, and much of the southern coastline was quickly overrun. The greatly outnumbered Imperial legions retreated across the Alik'r Desert in the now-famous March of Thirst. However, the surprising initial success of Lord Naarifin's attack led the Thalmor to believe that the Empire was weaker than they had thought. The capture of the Imperial City itself and the complete overthrow of the Empire thus became their primary objective over the next two years. As we know, the Thalmor nearly achieved their objective. It was only because of the Emperor's determined leadership during the Empire's darkest hour that this disaster was averted. During 4E 172, the Aldmeri advanced deeper into Cyrodiil. Bravil and Anvil both fell to the invaders. By the end of the year, Lord Naarifin had advanced to the very walls of the Imperial City. There were fierce naval clashes in Lake Rumare and along the Niben as the Imperial forces attempted to hold the eastern bank.
In Hammerfell, the Thalmor were content to consolidate their gains as they took control of the whole southern coastline, which was in fact their stated objective in the ultimatum delivered to the Emperor. Of the southern cities, only Hegathe still held out.
The survivors of the March of Thirst regrouped in northern Hammerfell, joined by reinforcements from High Rock. The year 4E 173 saw stiffening Imperial resistance in Cyrodiil, but the seemingly inexorable Aldmeri advance continued. Fresh legions from Skyrim bolstered the Emperor's main army in the Imperial City, but the Aldmeri forced the crossing of the Niben and began advancing in force up the eastern bank. By the end of the year, the Imperial City was surrounded on three sides - only the northern supply route to Bruma remained open.
In Hammerfell, Imperial fortunes took a turn for the better. In early 4E 173, a Forebear army from Sentinel broke the siege of Hegathe (a Crown city), leading to the reconciliation of the two factions. Despite this, Lady Arannelya's main army succeeded in crossing the Alik'r Desert. The Imperial Legions under General Decianus met them outside Skaven in a bloody and indecisive clash. Decianus withdrew and left Arannelya in possession of Skaven, but the Aldmeri were too weakened to continue their advance.
In 4E 174, the Thalmor leadership committed all available forces to the campaign in Cyrodiil, gambling on a decisive victory to end the war once and for all. During the spring, Aldmeri reinforcements gathered in southern Cyrodiil, and on 12th of Second Seed, they launched a massive assault on the Imperial City itself. One army drove north to completely surround the city, while Lord Naarifin's main force attacked the walls from the south, east, and west. The Emperor's decision to fight his way out of the city rather than make a last stand was a bold one. No general dared advise him to abandon the capital, but Titus II was proven right in the end.
While the Eighth Legion fought a desperate (and doomed) rearguard action on the walls of the city, Titus II broke out of the city to the north with his main army, smashing through the surrounding the Aldmeri forces and linking up with reinforcements marching south from Skyrim under General Jonna. However, the capital fell to the invaders and the infamous Sack of the Imperial City began. The Imperial Palace was burned, the White-Gold Tower itself looted, and all manner of atrocities carried out by the vengeful elves on an innocent populace.
In Hammerfell, General Decianus was preparing to drive the Aldmeri back from Skaven when he was ordered to march for Cyrodiil. Unwilling to abandon Hammerfell completely, he allowed a great number of "invalids" to be discharged from the Legions before they marched east. These veterans formed the core of the army that eventually drove Lady Arannelya's forces back across the Alik'r late in 4E 174, taking heavy losses on their retreat from harassing attacks by the Alik'r warriors.
During the winter of 4E 174-175, the Thalmor seem to have believed that the war in Cyrodiil was all but over. They made several attempts to negotiate with Titus II. The Emperor encouraged them in their belief that he was preparing to surrender; meanwhile, he gathered his forces to retake the Imperial City.
In what is now known as the Battle of the Red Ring, a battle that will serve as a model for Imperial strategists for generations to come, Titus II divided his forces into three armies. One army, with the legions from Hammerfell under General Decianus, was hidden in the Colovian Highlands near Chorrol. The Aldmeri were unaware that he was no longer in Hammerfell, possibly because the Imperial veterans Decianus had left behind led Lady Arannelya to believe that she still faced an Imperial army. The second army, largely of Nord legions under General Jonna, took up position near Cheydinhal. The main army was commanded by the Emperor himself, and would undertake the main assault of the Imperial City from the north. On the 30th of Rain's Hand, the bloody Battle of the Red Ring began as General Decianus swept down on the city from the west, while General Jonna's legionnaires drove south along the Red Ring Road. In a two-day assault, Jonna's army crossed the Niben and advanced west, attempting to link up with Decianus's legions and thus surround the Imperial City. Lord Naarifin was taken by surprise by Decianus's assault, but Jonna's troops faced bitter resistance as the Aldmeri counterattacked from Bravil and Skingrad. The heroic Nord legionnaires held firm, however, beating off the piecemeal Aldmeri attacks. By the fifth day of the battle, the Aldmeri army in the Imperial City was surrounded. Titus II led the assault from the north, personally capturing Lord Naarifin. It is rumored that the Emperor wielded the famed sword Goldbrand, although this has never been officially confirmed by the Imperial government.  An attempt by the Aldmeri to break out of the city to the south was blocked by the unbreakable shieldwall of General Jonna's battered legions. In the end, the main Aldmeri army in Cyrodiil was completely destroyed. The Emperor's decision to withdraw from the Imperial City in 4E 174 was bloodily vindicated. Lord Naarifin was kept alive for thirty-three days, hanging from the White-Gold tower. It is not recorded where his body was buried, if it was buried at all. One source claims he was carried off by a winged daedra on the thirty-fourth day.
The White-Gold ConcordatEdit
In 4E 175, the White-Gold Concordat was signed, officially ending the war. Two provisions in the treaty had serious impacts on what remained of the Empire. First, large portions of southern Hammerfell were ceded to the Aldmeri Dominion. The Redguards, outraged that the Empire paid for the treaty with their hard-won lands, began their own separate war with the Aldmeri. As a result, the Empire renounced Hammerfell as an Imperial province and abandoned the Redguards. With the resilience and strength that are characteristic of Redguard warriors, they fought back and defeated the Aldmeri. But, relations between Hammerfell and Cyrodiil 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, and some continued to worship Talos in secret. The Nords' anger over the White-Gold Concordat is one of the driving forces behind the current civil war in Skyrim.
The outcome of the Great War is simple -- 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 they have already begun to plan its execution.
The Dark Brotherhood of Cyrodiil was unintentionally 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. 
In the days before the Great War, the Blades began a shadow war against the Thalmor, believing that they were superior enough to handle the conflict. However, this arrogance led to the destruction of the Blades. In response to their aggression, the Thalmor systematically hunted down and exterminated the Blades in all parts of Tamriel. The remaining members of the Blades were in hiding in Skyrim at the outbreak of the Civil War. It is suggested that the Thalmor would have destroyed the Blades eventually in any case, since they saw the Blades' connection to Talos as a possible threat.
- It's implied by General Tullius before, at, or during the Civil War questline that the war with the Thalmor will resume but tells the player, "keep it between just the two of us".
- If the Stormcloaks win the war, both Ulfric Stormcloak and Galmar Stone-Fist will tell the player that after they have finished restoring Skyrim, they will take their armies to Summerset Isle and, "show those pointy-eared bastards that not every man is fit to be their slave".
- The Elder Scrolls V: Skyrim (Mentioned only)