The Siege of the North was a massive battle that was fought between the Northern Water Tribe and the Fire Nation. The Northern Water Tribe, led by their Chief, Arnook, and Aang, the Avatar, defended the Northern Water Tribe civilization from the navy and ground forces of the Fire Nation, led by Admiral Zhao. The battle ended with a huge defeat for the Fire Nation, and brought significant trouble to two of its survivors, Zuko and Iroh. It was fought mere months after Avatar Aang was awakened from one hundred years of frozen sleep within the Avatar State and months before the fall of Ba Sing Se.
Many months before the battle, Avatar Aang and his sky bison, Appa, were awoken from one hundred years of sleep within an iceberg by Sokka and Katara, two members of the Southern Water Tribe. After learning that he was the Avatar and after telling Aang about the 100-year war being waged by the Fire Nation, the four befriended each other and decided to take Aang to the Northern Water Tribe to find him a Waterbending master. As the Avatar, Aang had a duty to learn the four elements and to fight the Fire Nation invaders in order to restore peace to the world. After several months of traveling and evading Fire Nation pursuers across the Earth Kingdom, Aang and his friends reached the Northern Water Tribe and began learning the ways of advanced Waterbending.
However, the ambitious Firebender Admiral Zhao (formerly Commander; formerly Captain) had been tracking the Avatar across the Four Nations. Despite Aang being safe in the huge icy fortress of the Northern Water Tribe, Zhao was determined to achieve his goal. He amassed a huge invasion fleet, and recruited General Iroh to be his military adviser through treachery, and set off for the North Pole. Although this would be an incredibly difficult conquest, Zhao was confident. He even announced to Iroh that he had a strategy to eliminate Waterbending forever, as he knew about the mortal forms of the Moon and Ocean Spirits (Tui and La).
On a winter morning, Zhao and his Fire Navy armada had arrived on the gate of the Northern Water tribe forces using Empire-Class battleships, Admiral Zhao attacked by catapulting fireballs from his fleet to break through the icy defenses.
The Water Tribe countered by freezing and destroying Fire Nation ships. Zhao sent a single vessel to blast open the great ice walls defending the city, but this vessel was quickly disabled by Avatar Aang and the waterbenders. Avatar Aang flew into battle destroying at least a dozen of the imperial vessels before reaching exhaustion. Meanwhile Chief Arnook organized a secret mission to eliminate Admiral Zhao by placing his future son-in-law, Hahn to be to board the Fire Nation's flagship and capture or kill its commander. The effort turns out to futile due to the incompetence of the son-in-law to be.
After a day of heavy fire, Iroh advises Zhao to hold fire and resume the invasion the next day due to the moon augmenting the Waterbenders. Meanwhile Aang enters the Spirit World to gather the request the aid of the moon and ocean spirits to defeat the Fire Nation. After daybreak arrives Zhao resumes fire, within minutes breaches the outer gate of the Water Tribe and deploys a massive amount of infantry, cavalry, and tanks. The battle at this point seems almost hopeless for the Water Tribe with the Fire Nation advancing rapidly through their defenses. Aang meanwhile has been abducted by Prince Zuko.
On the night of the second day, the Waterbenders launch their counterattack. Drawing power from the full moon, they advance toward their enemy invaders and successfully begins to destroy their foothold on the beachhead. Realizing the difficult situation, Zhao reveals his plan to kill the moon spirit in order to render the Waterbenders useless forever.
Aang meanwhile has been rescued from Zuko by his friends and had learned the location of the moon and ocean spirits. Things do not go as planned as Avatar Aang and his friends make their stand at the koi pond. Despite their efforts and the efforts of General Iroh, Zhao is able to kill the Moon Spirit, stripping the Waterbenders of their powers.
At this point, Iroh abandons his allegiance to the Fire Nation and effortlessly strikes down Zhao's men, driving Zhao away from the sacred pond. Aang then enters the Avatar State and merges with the Ocean Spirit who in an act of revenge single-handedly destroys the invading forces and drives what is left of the Fire Nation fleet away and heavily damaged. Zhao himself was intercepted by Zuko who wanted revenge for Zhao's previous attempt to murder him.
Initially, the two firebenders fought on equal terms but Zuko gained the upper hand and overwhelmed the Admiral. Before their battle could be concluded, the Ocean Spirit, by this time separated from Aang, dragged Zhao to his death in a watery grave. The Moon Spirit itself was revived by Princess Yue's sacrifice restoring their bending powers. By this point, the few surviving Fire Nation warships abandoned the siege and retreated from the North Pole.
Not much is known about the remainder of the battle but it was possible that any surviving fire infantry were killed or captured. In the end the Water Tribe achieved what appears to be a Pyrrhic victory, having lost many civilians and large amounts of infrastructure. The leaders of the Water Tribe decided to re-open communication with all of their sister tribes and begins their plans of reunifying the tribes. This included sending a team of select healers and waterbenders to rebuild the ravaged southern tribe.
Although a tremendous victory, the loss of Zhao's Fire Navy fleet was not enough to end the War, and it was mere weeks after the battle that the Earth Kingdom city of Omashu fell to the enemy. However, the success of this battle ensured that the Northern Water Tribe survived the remainder of the War.
Although the loss of the invading fleet was a huge blow to the Fire Nation, the Fire Navy still maintained its power and most units still remained active, such as The Southern Raiders and their base on Whale Tail Island, and the blockade defending the Fire Nation.
The Siege of the North was possibly the world's greatest victory against the Fire Nation since the failure of the Siege of Ba Sing Se.
Fire Lord Ozai was furious about the defeat. Blaming his brother Iroh for its failure, he sent his daughter Azula to capture Iroh and Zuko.