The Iron Throne is forged, and the count begins
Aegon had the swords of his beaten enemies heaped and melted by dragonflame into a chair deliberately uncomfortable to sit in ÔÇö a king should never be at ease, he said. Anointed by the High Septon at Oldtown, he became the first king of the Seven Kingdoms, and the maesters began counting years from his crowning. Dorne, notably, was not conquered; it simply refused, for two centuries.
The fuller tale
Aegon had the swords of the men who had fought him gathered up ÔÇö a thousand of them, or so the story goes, though the chair has fewer ÔÇö and had Balerion work them into a seat. Twisted blades, barbs, a back like a wall of edges, and no comfort in it anywhere. A king should never sit easy, he said, and then he sat on it for thirty-seven years.
In the same year he was anointed a second time, at Oldtown, by the High Septon, which mattered more than the crown did. The Faith had every reason to declare against a conqueror who married both his sisters; it declared for him instead, and the Seven Kingdoms became a thing the gods had approved of rather than a thing three dragons had done. The maesters began counting years from that coronation, and everything in this chronicle is dated from it.
Two of the old kingdoms he did not win by fire. The North knelt because Torrhen Stark counted three dragons against thirty thousand men and did the sum honestly; he is called the King Who Knelt by people who have never had to make that decision. The Vale was given up by a regent after Visenya landed a dragon in the Eyrie's courtyard and let a small boy sit on it, which ended the discussion without anyone drawing a sword.
The one that got away
Dorne refused. It did not give battle, which was the point: the Dornish emptied their castles, went into the deserts and the mountains, and let Aegon burn empty rooms. Princess Meria Martell, eighty years old and blind, sent word that he could burn her country but he could not rule it, and she was right for two centuries. Rhaenys Targaryen and Meraxes were lost in the trying.
So the Seven Kingdoms began as six and a half, and the map lied about it for two hundred years. The chronicle's own map does not: Dorne stays its own colour until 187 AC.
Elsewhere in the wiki
 See the world as it stood in 1 AC What became of it  if an ending is written, it is here. Spoilers.
The chronicle records no ending here ÔÇö or none the maesters have set down.