Rook's Rest ÔÇö dragon against dragon
The war's first great lesson, taught over a modest Crownlands castle: a dragon can be killed, and the thing most likely to kill it is another dragon. Both sides learn that their ultimate weapon is not invulnerable, and neither of them draws the sensible conclusion.
The fuller tale
Rook's Rest was a modest castle in the Crownlands whose lord had declared for the queen, and taking it should have been a footnote. What happened over it changed how the whole war was fought.
The greens set a trap and it worked: King Aegon II came on Sunfyre, his half-brother Aemond came on Vhagar, and when Rhaenys Targaryen flew in on Meleys to break the siege she found herself facing two dragons instead of one. She fought anyway. Meleys was the swiftest dragon in the world and Rhaenys had ridden her for forty years, and she took Sunfyre down with her.
All three came out of the sky. Rhaenys and Meleys were killed. Sunfyre was so badly broken that Aegon was pulled out of the wreck burned half to ruin, unable to walk, and was carried out of his own war for a year. Only Aemond and Vhagar flew away, which made Aemond the most dangerous thing in Westeros and put the government in the hands of a nineteen-year-old with one eye and a grudge.
The lesson nobody learned
Before Rook's Rest, both sides had treated their dragons as an argument that ended discussions. After it, everyone in Westeros had watched three dragons go up and one come down. A dragon could be killed, and the thing most likely to kill it was another dragon.
The rational response would have been to stop risking them. Instead both sides concluded that the answer was to bring more, which is how the largest concentration of dragons in history was spent in twenty-four months. The Queen Who Never Was ÔÇö passed over for the throne twice, at the Great Council and before it ÔÇö is the first great casualty, and she chose the manner of it.
Elsewhere in the wiki
 See the world as it stood in 129 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.