The War of Conquest, also called the Targaryen Conquest, was a major war that resulted in the unification of six of the Seven Kingdoms of Westeros under the invading forces of House Targaryen. The war took place approximately 300 years before the War of the Five Kings.
The Targaryen family had moved from Valyria to the island stronghold of Dragonstone some years prior to the Doom. When the Doom took place, the Targaryens were the only family with dragons to survive the cataclysm. During the following century, the remnants of the Valyrian empire tore themselves apart, leading to the formation of the Free Cities and the liberation of Ghiscar and the cities of Slaver's Bay.
War in the Riverlands
Roughly one century after the Doom, Aegon Targaryen and his sister-wives, Rhaenys and Visenya, led a small army in landing at the mouth of the Blackwater Rush. They established a redoubt that eventually became the city of King's Landing. Aegon's forces made war upon Harren the Black of House Hoare, the ruler of the Iron Islands and Riverlands. Aided by a popular uprising against the ironborn led by House Tully, Aegon eventually bottled Harren up in his castle of Harrenhal. When Harren refused to surrender, Aegon had his dragon, Balerion, burn the castle with such ferocity that the towers melted like candles, roasting Harren and the other remaining members of House Hoare alive. Afterwards, Aegon raised the Tullys to become the Lords Paramount of the Trident and commanded the ironborn to elect a new ruling house. They chose House Greyjoy of Pyke, who swore fealty to Aegon and became the rulers of the Iron Islands.
War in the Stormlands
At the same time, Aegon's bastard half-brother, Orys Baratheon, led an army against Argilac the Arrogant of House Durrandon, the Storm King. Argilac chose to give battle in the field and was defeated and killed, eliminating House Durrandon. Orys took his castle of Storm's End, his sigil and words, and Argilac's daughter Argalia to wife. Aegon rewarded him by naming him Lord of Storm's End and allowing him to found House Baratheon.
War in the West
The forces of House Lannister and House Gardener combined and marched to meet Aegon's army in battle. At a field in the Reach that became known as the Field of Fire, Aegon unleashed all three dragons and decimated the enemy host within minutes. House Gardener was destroyed. Their stewards, the Tyrells, surrendered the Reach to Aegon and were named Lords Paramount of the Reach. The Lannisters also bent the knee and became Lords Paramount of the Westerlands.
Stand-off with the North and the Vale
House Arryn prepared for war. The first year of the war, they had repelled a Targaryen invasion of The Vale with Braavosi aid. Upon the defeat of the Lannisters and the Gardeners, the Arryns mobilized their army in anticipation of a second invasion. However, upon seeing a dragon, boy-king Ronnel Arryn surrendered peacefully in exchange for a ride, and no combat took place.
House Stark also mobilized for war, but through diplomacy and the demonstration of Aegon's superior forces, they bent the knee peacefully. House Stark thus became the only great house to emerge from the war without participating in combat at all.
Invasion of Dorne
Aegon's forces made an abortive attempt to conquer Dorne, but the Dornish chose to melt away into the mountains and deserts and pursue guerrilla tactics rather than give open battle, where they would be vulnerable to dragonfire. Aegon eventually decided to allow Dorne to retain its independence, ending the War of Conquest.
In the books
In A Song of Ice and Fire, the War of Conquest unfolds much the same as in the novels. However, Martin later expanded the detail of the war significantly in The World of Ice and Fire.
In this book, it was revealed that the Targaryens fled Valyria after Daenys Targaryen had a prophetic vision of the Doom. Her father, Aenar, led an exodus to Dragonstone (which had been a Valyrian outpost for two centuries at that time). The prophecy was proved correct when the Doom took place, twelve years later.
A century later, Aegon the Conqueror (born 27 years before the Conquest) began making his plans. He visited Westeros in secret and gathered intelligence on the loyalties and divisions between the noble houses. However, the war actually began due to an act of hostility by the Storm King. Argilac Durrandon proposed an alliance with Aegon against the ironborn, as he feared that once Harren Hoare had completed Harrenhal he would invade the Stormlands. Argilac proposed marrying his daughter, Argella (not Argalia, a name invented for the TV series by Bryan Cogman), to Aegon to cement the deal. Aegon, already married (twice over), instead suggested that Argella marry his half-brother Orys. Argilac, furious had Aegon's envoy's hands cut off and sent back to him, saying those were the only hands a bastard could expect of him.
Aegon sent ravens to the rulers of the Seven Kingdoms, demanding their loyalty and threatening them with destruction. Sharra Arryn, Queen-Regent of the Kingdom of the Mountain and the Vale, instead offered a mutual alliance on the condition that Aegon marry her and make her son, the infant King Ronnel Arryn, his heir. Princess Mariya Martell of Dorne offered a military alliance aimed at destroying the Storm King, but nothing more. Aegon ignored both offers.
Aegon's army of less than 3,000 troops landed at the mouth of the Blackwater, where he declared himself King of the Seven Kingdoms. In a rapid campaign of movement, his forces quickly captured Rosby, Stokeworth, Duskendale and Maidenpool. He then split his forces in three: the Targaryen fleet commanded by Daemon Velaryon would sail north, escorted by Visenya and her dragon, to land at Gulltown and begin the conquest of the Vale. Orys and Rhaenys would lead an army south to deal with Argilac. Aegon himself would march on Harrenhal.
The Targaryen fleet was intercepted by the Arryn fleet out of Gulltown and a furious sea battle took place. The Targaryen fleet was destroyed, but Visenya was able to burn most of the Arryn fleet in retaliation, resulting in a stalemate. It was a strategic victory for the Arryns, however, as they prevented an invasion of the Vale.
Aegon's invasion of the Riverlands was spearheaded by him on dragonback, burning the ironborn longships on the rivers and on Gods Eye (and killing several of Harren's sons). Impressed, Lord Edmyn Tully of Riverrun led a popular uprising against the ironbron. Houses Tully, Bracken, Blackwood, Mallister and Frey marched on Harrenhal and joined forces with Aegon. However, even this alliance only gave him 8,000 troops, far too few to take the castle. Harren insulted Aegon and refused to surrender. Aegon burned the castle with Balerion, killing Harren. The ironborn surrendered.
Hearing of Harren's fate, Argilac refused to be slaughtered like an animal and instead chose to give open battle. When a massive thunderstorm erupted as the armies assembled, Argilac took it as a sign and attacked. Outnumbering the Targaryen host 2-1, Argilac's forces gained the upper hand and outflanked Orys's forces. However, Rhaenys and her dragon Meraxes entered the fray, destroying part of Argilac's army while Orys engaged Argilac in personal combat and slew him. Argella declared herself the Storm Queen and tried to fight on, but her own bannermen betrayed her and handed her over to Orys. Orys treated her with kindness and accepted the surrender of Storm's End.
The Lannisters and Gardeners marched against Aegon's forces. Aegon regrouped with his sister-wives at Stoney Sept before marching south to the Reach. They destroyed the Host of the Two Kings on the Field of Fire and accepted the surrender of their bannermen. Aegon meant to press on to Dorne with his combined host, but then learned that King Torrhen Stark and 30,000 northmen were marching south. He rushed north to meet them at the Trident. Torrhen had planned to give battle, even entertaining a plan put forward by his bastard half-brother Brandon Snow to assassinate the dragons through stealth, but upon seeing Aegon's vaster numbers and dragons he knew he could not subject Westeros to another field of fire. He sent three maesters to negotiate a surrender and then knelt before Aegon, who named him Lord of Winterfell and Warden of the North.
The Arryns had also assembled a mighty host at the Bloody Gate and were preparing for war. However, choosing a different tactic, Visenya flew her dragon straight into the courtyard of the Eyrie, where the young boy-king Ronnel was playing. When his mother came out of the castle, she found that Visenya had already negotiated the peaceful submission of House Arryn with Ronnel in return for being given a ride on her dragon. The Arryns also bent the knee to Aegon.
Rhaenys led an army into Dorne, only to find the holdfasts empty and the armies of Dorne scattered into small bands which constantly harassed her supply lines. Frustrated, she flew to Sunspear to confront Princess Mariya Martell, the so-called Yellow Toad of Dorne. Mariya, eighty years old, blind and almost bald, defied Rhaenys to her face, telling her that Dorne would never surrender. Fearing that the cost of taking Dorne would be ruinous, Aegon agreed to allow it to remain independent.
Aegon triumphantly entered Oldtown and was proclaimed King of Westeros, ending the War of Conquest after two years.