- "There is only one war that matters. The Great War. And it is here."
- ―Jon Snow at the Parley in King's Landing
The Great War is the ultimate culmination of the long-simmering Conflict Beyond the Wall: the war between the living and the dead. It is the second war waged by the White Walkers against humanity, the first having occurred thousands of years ago during the first Long Night in the form of the War for the Dawn.
The opening stages of the Great War are a war of attrition between the Night King, his legions of White Walkers, and their army of the dead against the living peoples of Westeros. Currently, they are opposed by the Night's Watch, the North, the Brotherhood Without Banners, the Free Folk, and the Vale, all under the leadership of Jon Snow, the King in the North. Jon is later joined by Daenerys Targaryen, who leads the forces of House Targaryen along with House Lannister under her hand, Tyrion Lannister. Jaime Lannister also abandons his sister Cersei to travel north to Winterfell to join the military forces of the North after she reveals her intentions to Jaime to hold back the southern armies.
Although expected to threaten all of Westeros and possibly the world, the Great War at its beginning is centered on the North, as the southern kingdoms are in disbelief of the White Walkers and are more preoccupied with the aftermath of the War of the Five Kings and Daenerys Targaryen's invasion of Westeros, another long-awaited conflict which began around the same time the North began preparations for the coming Great War.
The White Walkers first descended upon Westeros during the Age of Heroes 8,000 years ago, bringing with them a cold and terrible winter and a dark night that never ended. The Night King had been created thousands of years prior by the Children of the Forest, the natives of Westeros, with dragonglass to combat the invading First Men in the Dawn Age, but the Children and the First Men came together in War for the Dawn to fight against their common enemy. The White Walkers and their wights, which had been raised from those that they slaughtered, were defeated and driven back to the far north while the Wall was raised and the Night's Watch formed to prevent the White Walkers from returning. Many were trapped north of the Wall, becoming the Free Folk, while those south of the Wall began to form kingdoms.
As the many centuries passed, most of Westeros came to see the magical White Walkers and Children of the Forest as nothing more than legends and scary fairy tales, especially as the Andals and the Rhoynar had never encountered them on Westeros since their migrations came thousands of years after the Long Night. However, the White Walkers eventually rose from the ice again beyond the Wall around the time of Robert's Rebellion. While the southern Seven Kingdoms were preoccupied with their own civil war and largely did not believe in the White Walkers, the Free Folk, who were being slaughtered by the White Walkers to amass a new army of the dead, were attempting to cross the Wall while the Night's Watch was seeking out the truth and fighting against their old wildling enemies in the Conflict Beyond the Wall.
However, the Night's Watch, under Lord Commander Jon Snow, let the wildlings cross the Wall after realizing the existence of the White Walkers and the threat that they posed. They come south of the Wall after the attempted rescue mission at Hardhome. This decision led to a mutiny but Jon was resurrected and freed from his life long vows to the Night's Watch. Seeking to unite the North against the White Walkers, Jon, himself the alleged illegitimate son of Ned Stark, joined with his half-sister Sansa Stark to save their younger brother Rickon Stark and reclaim their home Winterfell from House Bolton, who had previously usurped House Stark from the North after the Red Wedding. Ramsay Bolton and his allies were defeated and killed at the Battle of the Bastards, and in the aftermath of the battle, Sansa became the new Lady of Winterfell while Jon was declared the King in the North by the lords of the North and the Vale at Winterfell. Around the same time, Daenerys Targaryen, the only known survivor of the once royal House Targaryen which had been ousted during Robert's Rebellion, begun setting sail for Dragonstone with her allies, vassals, and three dragons, the first born in the world in over a century. At this time, Bran Stark discovered via Greensight that Jon, unbeknownst to all including Jon himself, is of both Stark and Targaryen lineage as the secret son of Lyanna Stark, Ned Stark's sister, and Rhaegar Targaryen, Daenerys's eldest brother, both of whom died in Robert's Rebellion.
Melisandre meets Daenerys Targaryen at Dragonstone, who has begun planing her invasion of Westeros to take back the Iron Throne from Cersei Lannister, who seized the crown after the extinction of the Baratheon dynasty that had been formed at the end of Robert's Rebellion. She urges Daenerys to summon Jon Snow to Dragonstone, believing that they both have a role to play in the Great War for the dawn, later referring to them as "ice and fire" to Varys. Tyrion Lannister, Daenerys's Hand, informs Daenerys that he met and trusts Jon and that the North could make a valuable ally, and Daenerys agrees. They eventually meet at Dragonstone as Jon and Ser Davos Seaworth are brought to the throne room. Jon refuses to bend the knee but attempts to persuade her of the existence of the White Walkers and the realness of their threat. Daenerys and Tyrion are skeptical, but she lets him stay as a "guest" at Dragonstone. Later, Tyrion and Jon discuss the army of the dead, and Tyrion, despite his skepticism, trusts Jon and persuades Daenerys to allow Jon to mine the dragonglass. Tyrion explains that if Jon is wrong - the dragonglass is worthless, and Daenerys does not need it anyway (she did not even know it was there in the first place, so it is nothing to her); allowing Jon to mine it may result in an alliance.
Before beginning the mining operations, Jon leads Daenerys down to the caves where he shows her the dragonglass. He leads her further, revealing carvings that he explains were engraved by the Children of the Forest. Daenerys wonders if they were made before the coming of the First Men to Westeros, but Jon shows her the Children's depictions of the Children and the First Men, despite their conflicts, fighting together against a common enemy: the Night King and his White Walkers. Daenerys, her trust in Jon growing, agrees to fight for the North... when Jon bends the knee. Jon tells her that his bannermen will not accept another southern ruler, but Daenerys is confident that his people will follow him.
In the Chamber of the Painted Table, Jon receives a raven from his half-brother Bran Stark, a warg who had been trained in greensight by the Three-Eyed Raven, revealing that he and Arya Stark are alive and at Winterfell, yet also warning that the Night King is on the march to Eastwatch-by-the-Sea. Tyrion proposes that the best way to prove the army of the dead to Cersei Lannister and Westeros is to capture a wight. Jon travels to Eastwatch with Ser Jorah Mormont of Daenerys's Queensguard, Ser Davos, and Gendry where they meet with Tormund as well as three prominent brothers of the Brotherhood Without Banners: Lord Beric Dondarrion, Thoros of Myr, and Sandor Clegane. Despite their differences from the past, they all agree to march beyond the Wall with Jon on his Wight Hunt. Beyond the Wall, the group is able to come together and put aside their differences through conversation while marching. They succeed in capturing a wight despite being attacked by a wight snow bear earlier but draw the attention of a large contingent of the army of the dead, forcing them onto an island in the middle of a frozen lake. Gendry runs back to Eastwatch, where he has Ser Davos send a raven to Dragonstone asking for Daenerys's help. Though Tyrion advises that she stay behind out of concern for her safety and their cause, Daenerys flies to the far north with her three dragons, Drogon, Rhaegal, and Viserion, and saves the surviving members of Jon's group. However, the Night King is able to kill Viserion, and Jon is left behind when two wights drag him beneath the frozen lake. Nonetheless, Jon escapes and is later saved by his long lost uncle, Benjen Stark, who sacrifices himself to allow Jon to flee back to Eastwatch. On a Targaryen ship en route to King's Landing for a parley, Jon privately pledges his loyalty to Daenerys Targaryen while beyond the Wall, the Night King reanimates the corpse of Viserion.
The Parley in King's Landing is held at the Dragonpit atop Rhaenys's Hill, where Jon Snow presents the captured wight to the gathered attendants: Cersei Lannister, Euron Greyjoy, and their entourage. Jon warns them that only one war matters now: the Great War. However, things turn sour when Jon publicly declares for Daenerys his queen, confessing to having already pledged his loyalty to her. Nonetheless, Tyrion is able to persuade his sister to fight alongside the Targaryen and Stark forces in the coming Great War. Daenerys and Jon sail to White Harbor with their gathered forces while the Dothraki ride hard up the Kingsroad. Secretly, however, Cersei later confesses to her brother Jaime while planning the Lannister march north with their gathered bannermen that she plans on betraying the Targaryens and the Starks, and that Euron, who previously announced he was fleeing back to the Iron Islands, is ferrying the Golden Company from Essos to Westeros. When Cersei threatens Jaime's life after repeatedly mocking him, Jaime takes his leave and rides north alone. Meanwhile, at the far north, Tormund, Beric Dondarrion, and the garrison at Eastwatch watch on as the White Walkers lead the wight army out of the Haunted Forest and the Night King attacks the Wall with the undead dragon Viserion. A massive breach is created where Eastwatch once stood, nullifying the magic of the Wall and allowing the White Walkers and their wight army to cross into southern Westeros once more.
In the books
In the A Song of Ice and Fire novels, the Great War, while heavily referenced as an impending event, has not yet begun in earnest. The conflict between the Night's Watch and the Free Folk is still in progress, and Stannis Baratheon still lives, and is currently marching on Winterfell in an attempt to retake the North from House Bolton.