In A Game Of Thrones (I think, it may be one of the later books), Robert's Rebellion is briefly recapped and it is said that when Robert won, he stripped lands from those who opposed his claim during the rebellion.
In addition, there was the epic battle at the Trident where Rhaegar was killed by Robert.
Does it ever address in the books who supported the Targaryens during the war?
We know that the Vale (House Arryn), the Riverlands (House Tully) and the North (House Stark) supported the Stormlands (House Baratheon) in the rebellion, so from the seven kingdoms there are only the Westerlands (House Lannister), Highgarden (House Tyrell) and Dorne (House Martell) remaining.
The rest here is not based on memory so let me know if it's incorrect.
From what I gathered, the Westerlands pretty much kept out of the war. They covertly supported the Throne (which is how Tywin managed to gain access to King's Landing when it was sacked), but I don't think they ever openly opposed the Rebellion in battle.
Dorne probably supported the Throne because Elia was married to Rhaegar at the time.
How about Highgarden? Was this ever addressed? Or did the Targaryens hire mercenaries (like the Golden Company or the Second Sons) to fight on their behalf?
It seems like 4 houses going up against 2 wouldn't be much of a battle. Obviously Robert won, but I assumed it was a close contest.
Were the lands that Robert stripped just Dragonstone and its supporting Houses from the Targaryens?