The Young Dragon takes Dorne ÔÇö briefly
Daeron I conquered Dorne at fourteen with strategy instead of dragons, wrote a book about it, and lost it again to rebellion within four years, along with his own life and some fifty thousand men. Dorne would be joined to the realm only two reigns later, by the far duller method of a marriage.
The fuller tale
Daeron I came to the throne at fourteen, and the first thing he did was announce that he would finish what Aegon the Conqueror could not. He had no dragons. He had a plan, which was better: he sent the Tyrells and the Baratheons at the Prince's Pass and the Boneway to be seen, and took the main host himself over the Stepstones and up through the passes nobody used, and was in Sunspear before the Dornish had finished counting the armies they could see.
It took him summer and autumn. He wrote a book about it afterwards, the Conquest of Dorne, which is still read at the Citadel and is the most confident thing any Targaryen ever put on paper.
It came apart because taking Dorne and holding Dorne are different problems and always have been. The garrisons were picked off, the hostages taken to King's Landing were an insult rather than a security, and the countryside simply refused. Ten thousand men were lost holding a country that had already surrendered. Daeron went back south under a peace banner to settle it, and was killed there ÔÇö treacherously, say the histories written in King's Landing.
Fifty thousand men
That is the figure given for the whole adventure, dead in four years, for a province that was handed to the crown two reigns later by a marriage that cost nothing. Daeron was eighteen when he died and is remembered fondly, which is what happens to young kings who die winning.
His brother Baelor walked barefoot to Dorne to make peace and freed the hostages by climbing to their cell through a pit of vipers, which is either sanctity or a different kind of madness. Between the two of them the crown spent a decade discovering that Dorne is not a military problem.
Elsewhere in the wiki
 See the world as it stood in 157 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.