The Kingdom of the Iron Islands is an independent kingdom in Westeros.
The first King of the Iron Islands was the Grey King, is a legendary ironborn King of the Iron Islands. Blessed by the Drowned God, the Grey King was said to have waged war against the Storm God himself and slain Nagga, the first sea dragon, using its bones to build his hall. Legend has it that he also took a mermaid as his wife and lived for a thousand years. House Greyjoy claims descent from him.
The Kingdom of the Iron Islands was re-founded under the reign of King Balon Greyjoy during the Greyjoy Rebellion. The kingdom ceased to exist when Balon Greyjoy lost the rebellion and bent the knee to King Robert Baratheon.
War of the Five Kings
The Kingdom of the Iron Islands was re-founded once more by Balon Greyjoy during the War of the Five Kings.
War for the North
In 301 AL, the Ironborn, under the commander of Princess Yara Greyjoy, assault the Dreadfort in order to resecue Theon Greyjoy, but they fail their mission. Later that same year, the Ironborn holding Moat Cailin surrenders to Bolton army and all surrendering Ironborn are flayed alive.
Assassination of King Balon Greyjoy
In 303 AL, Euron Greyjoy decides to return to the Iron Islands, confronts his brother, Balon, in Pyke and throws Balon off the bridge and into the ocean to drown to death. Shortly before the Kingsmoot, Theon returns to Pyke. Following a tense reunion with his sister Yara, Theon mollifies her by pledging to support her claim to the Salt Throne.
Kingsmoot upon the death of Balon Greyjoy
At the Kingsmoot, Yara gains considerable support from the assembled Ironborn by promising to restore the former glory of the Ironborn. When a delegate derides Yara for being a woman, Theon springs to her defense by pointing out that she is a proven warrior, reaver, and Ironborn. Many in the crowd call Yara to be crowned as the first Ironborn Queen.
However, their uncle Euron puts forth his claim to the Salt Throne. He attacks his nephew by highlighting his defeats and emasculation while mocking his niece as a woman. When Yara accuses Euron of murdering their father, Euron openly admits to the act but claims that his actions were justified because of Balon's incompetence. In response, Theon points out that Euron was gallivanting on the high seas for several years while Yara was commanding men and ships. When Yara promises to build a mighty fleet to conquer the mainland, Euron counters by proposing to forge an alliance with Daenerys Targaryen to conquer the mainland with her dragons. Euron wins the Kingsmoot by telling the Ironborn that he has paid the iron price.
House Greyjoy divided
While Euron is baptized by his brother Aeron in the sea, Yara, Theon, and their supporters flee on the best ships in the Iron Fleet. Euron drowns but is revived on the beach. Aeron then crowns Euron with the driftwood crown. After learning that his nephew and niece have escaped, Euron orders his subjects to build the biggest fleet the world has seen. He also vows to hunt down and kill his niece and nephew.
Yara and Theon take the Iron Fleet to first Volantis and than Meereen. Yara and Theon tell Daenerys about Euron's plan and stated they wanted to form an alliance with her. In exchange for their support and a pledge to end the Old Way, Daenerys will recognize the independence Kingdom of the Iron Islands and Yara as Queen of the Iron Islands. Yara and Daenerys agree to the deal.
The Seven Kingdoms under the reigns of King Robert I Baratheon, King Viserys III Targaryen, King Joffrey I Baratheon, King Stannis I Baratheon, King Renly I Baratheon, King Tommen I Baratheon, and Queen Cersei I Lannister recognized the territory of the Iron Islands as a constituent region of the Seven Kingdoms.
Until 303 AL, the Seven Kingdoms under the reign Queen Daenerys I Targaryen also recognized the territory of the Iron Islands as a constituent region of the Seven Kingdoms. In 303 AL, after the Second Siege of Meereen, Queen Daenerys I Targaryen agreed to support the rights of Yara Greyjoy. She did so at the express stipulation that the Ironborn cease reaving (the same law enforced in the first Targaryen conquest, and symbolic of their rule by the Iron Throne). During the exchange, Tyrion Lannister explicitly advised that allowing an independent realm would be a dangerous precedent.
It is unclear if the Kingdom of the North under King Jon I Snow recognizes the independence of the Kingdom of the Iron Islands. The Kingdom of the Iron Islands and the Kingdom of the North under King Robb I Stark actively fought each other during the War of the Five Kings.