LogFAQs > #922833587

King Aerys was convinced that Rhaegar was leading a plot against him and he shows up at Harrenhal precisely for that reason. But in the years prior to the tourney, you can already see the seeds of an alliance forming among the Great Lords.

Eddard and Robert are fostered by Jon Arryn, forging ties between Houses Arryn, Stark and Baratheon
Lyanna is betrothed to Robert, cementing the ties between House Stark and Baratheon
Catelyn is betrothed to Brandon, creating a tie between House Tully and Stark
And Hoster Tully attempts to betroth Lysa to Jaime Lannister - which Aerys quashes by appointing Jaime to the Kingsguard, the event that leads Tywin to resign as Hand of the King.

So even prior to Harrenhal, the alliance that will fight for Robert during the Rebellion is already in place, and had everything gone to plan House Lannister would have been included as well. The only mainland houses missing are the Tyrells, who were staunch Targaryen loyalists, and the Martells, who were tied to Prince Rhaegar through his marriage to Elia.
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Scoop #5: Borash tried to interview Bigfoot, but he ran away, Borash suggested if we dance hard enough he will come back