If you mean war as in war between separate Kingdoms instead of a Civil war, that will be pre-Conquest era. The closest case is the Lannister-Hoare war fought for avenging the Lannister Princess who had been mutilated by the Ironborn King (Who happened to be son of that Princess). Lannisters didn't intend to conquer or rule Iron Islands in the aftermath and in fact withdrew after events explained below. As soon as the Lion King's aunt had been avenged, and the Ironborn King hanged, their mission was completed as far as they were concerned.
It's featured in The World of Ice and Fire.
The belligerent factions were:
- Westerlanders under House Lannister
- Ironborn under House Hoare nominally and Ironborn clergy de-facto
The important figures in the struggle were:
- King Harmund the Handsome
- the Shrike
- King Hagon Hoare
- Queen Mother Lelia Lannister, Princess of the Rock
- Lelia's nephew, unnamed King of the Rock
- Ser Aubrey Crakehall, A Lannister General
The Twelve Years War 1
The trouble started with Hagon's father, King Harmund II Hoare. He was a scholar and loved traveling. So all in all, he wasn't the typical Ironborn King. This could be attributed to the fact that he was raised as a ward by the Lannisters at Casterly Rock. When his father died, his foster-Father the Lannister King gave him the hand of his daughter Princess Lelia Lannister and the Young King returned to the Isles with his Greenlander Queen.
His son Harmund the Haggler shared his love of reading, and became
renowned as a great traveler. He was the first king of the Iron
Islands to visit the green lands without a sword in his hand. Having
spent his youth as a ward of House Lannister, the second Harmund
returned to Casterly Rock as a king and took the Lady Lelia Lannister,
a daughter of the King of the Rock and "the fairest flower of the
west," for his queen. On a later voyage he visited Highgarden and
Oldtown, to treat with their lords and kings and foster trade.
TWOIAF - The Black Blood
He also raised up his sons, Harmund Junior and Hagon, in light of the Seven, instead of as followers of the Drowned God. While there were no rebellions against him, his son's one wrong move proved to be the last straw.
His son declared that henceforth Ironborn raiders would be hanged as pirates, thralldom would be outlawed and Saltwives would be considered mistresses and their children bastards, probably under influence from his mother. Ironborn rose in Rebellion to oust this infidel King.
His own sons were raised in the Faith, or King Harmund's own peculiar
version of it. Upon his death, the eldest of them ascended the throne.
Harmund the Handsome (influenced, some say, by his Lannister mother, the Dowager Queen Lelia) announced that henceforth reavers would be
hanged as pirates rather than celebrated, and formally outlawed the
taking of salt wives, declaring the children of such unions to be
bastards with no right of inheritance. He was considering a measure
to end the practice of thralldom on the isles as well when a priest
known as the Shrike began to preach against him.
Other priests took up the cry, and the lords of the isles took heed. Only the septons and their followers stood by King Harmund, and he was
overthrown within a fortnight, almost bloodlessly. What followed was
far from bloodless, however. The Shrike himself tore out the deposed
king's tongue, so he might never again speak "lies and blasphemies."
Harmund was blinded as well, and his nose was cut off, so "all men
might see him for the monster he is."
TWOIAF - The Black Blood
King Hagon took the throne when his elder brother Harmund was deposed by a religious uprising of the Ironborn. He allowed the rebels to mutilate his mother, the Princess Lelia Lannister of the Rock.
King Hagon, soon to be known as Hagon the Heartless, even permitted
the mutilation of his own mother, Queen Lelia, the Lannister "whore"
who was blamed by the Shrike for turning her husband and sons away
from the true god. Her lips, ears, and eyelids were cut off and her
tongue ripped out with hot pincers, after which she was bundled onto a
longship and returned to Lannisport.
TWOIAF - The Black Blood
When he sent his mutilated mother back to the Rock, her nephew the King of the Rock called his banners and declared war to avenge his aunt.
The King of the Rock, her nephew, was so angered by this atrocity
that he called his banners.
TWOIAF - The Black Blood
Ser Aubrey Crakehall, the General of the Westerlander Army defeated the Ironborn and hanged Hagon after mutilating him the same way as he had mutilated the Westerlander Princess.
The war that followed left ten thousand dead, three-quarters of them
ironborn. In its seventh year, the westermen landed on Great Wyk,
smashed Hagon's host in battle, and captured his castle. Hagon the
Heartless was mutilated in the same fashion as his mother before being
hanged. Ser Aubrey Crakehall, commanding the Lannister armies, ordered
that Hoare Castle be razed to the ground, but as his men were
looting, they came upon Harmund the Handsome in a dungeon. Crakehall
briefly considered restoring Harmund to his throne, Haereg claims, but
the former king was blind, broken, and half-mad from long confinement.
Ser Aubrey granted him "the gift of death" instead, serving Harmund a
cup of wine laced with milk of the poppy.
TWOIAF - The Black Blood
That officially ended the War. Ser Aubrey however didn't return home and declared himself King of the Iron Islands. The Lannister King however didn't approve of this and therefore Westerlander Forces withdrew to Westerlands. Ironborn overthrew Ser Aubrey within months.
Then, in an act of mad folly, the knight decided to claim the
kingship of the Iron Islands for himself. This pleased neither the
ironborn nor the Lannisters. When word reached Casterly Rock, the
king called his warships home, leaving Crakehall to fend for himself.
Without the power and wealth of House Lannister to prop him up, "King
Aubrey" saw his power crumble quickly. His reign lasted less than
half a year before he was captured and sacrificed to the sea by the
Shrike himself.
TWOIAF - The Black Blood
However for some reason, possibly the resumption of hostilities by the Ironborn to avenge their humiliating defeat (Because they are really stupid when it concerns their pride), the Lannisters and Ironborn were soon at war again albeit in a desultory fashion and they remained at war for the next five years after which they signed a peace accord presumably in favor of Lannisters.
The war between the ironborn and the westermen continued in a desultory fashion for five more years, finally ending in an exhausted
peace that left the Iron Islands impoverished, burned, and broken.
The winter that followed was long and harsh, and is remembered on the
isles as the Famine Winter. Hake tells us that three times as many
ironmen perished of starvation that winter than had died in the
battles that preceded it.
TWOIAF - The Black Blood
1. This is not the official name of that war but I am calling it that because it took Lannisters seven years to smash the Ironborn and it took the Ironborn further five years to realize they were defeated.
