What was Aegon's Conquest?

How could three riders conquer a continent?

Because the invaders brought weapons no army in Westeros could match. The force that crossed with Aegon counted fewer than 1,600 men, far too few to conquer anything by ordinary means, but he and his sisters brought three dragons, Balerion, Vhagar and Meraxes. The size of the army hardly mattered; what the lords of Westeros had to weigh was what the dragons could do to a castle or a host.

The landing itself shows the confidence of the plan. Aegon came ashore by three hills at the river's mouth and built a wooden fort, the Aegonfort, on the highest of them as his first claim to the mainland. He did not need a fortress or a port to start a conquest; he needed a place to stand while the lords of the realm decided whether to fight him.

Why did some kingdoms fight and others kneel?

The ones that fought first taught the others. At the Field of Fire, King Loren Lannister and King Mern Gardener met Aegon in battle and dragonfire broke their armies; Mern died there. The great castle of Harrenhal was burned by dragonfire. After those two lessons, defiance had a known price, and the remaining lords could count.

Torrhen Stark, King in the North, marched south with his army, reached the Trident, and found Aegon there ahead of him, with three dragons and a host of forty-five thousand at his back. He knelt, his house went on ruling the North as Wardens for the new king, and he was remembered as the King Who Knelt. It is a name that sounds like an insult and works as a judgement on the alternatives: the kings who refused to kneel are remembered for what burned.

Why did Dorne hold out?

The histories are clear about the outcome. Dorne, ruled by House Martell from Sunspear, was the one kingdom the Conquest did not take, and it remained independent while the other six came under the Targaryen crown. The conflict did not end there: the fighting is counted as the First Dornish War, and a second followed at the start of Aenys I's reign in 37 AC.

Two more Dornish wars are counted during the long reign of Jaehaerys I, which shows how far from settled the matter was. The realm was called the Seven Kingdoms from the start, but for generations the name described an ambition rather than a fact, and the Iron Throne's authority stopped at the Dornish border no matter how many dragons stood behind it.

What did the Conquest leave behind?

A throne, a calendar and a habit of mind. Aegon had the Iron Throne made from the swords of the enemies he had defeated, fused by dragonfire, so the seat of the new realm was itself a monument to the war. And the maesters began to date events from his coronation, counting years before and after it, which is why every date in the histories carries BC or AC.

The habit of mind is the idea of the Seven Kingdoms as one realm. Before Aegon, the lords of Westeros thought of themselves as subjects of separate crowns; after him, even the rebels who overthrew his descendants fought over his throne rather than breaking it up. The list of monarchs that begins with Aegon runs, in the show, right up to the moment the throne itself is destroyed.