The Third Border Wars was the first civil war in the history of the Seven Kingdoms, fought between bordermen rebels and Targaryen/Baratheon loyalists. The war erupted after Aegon the Conqueror successfully conquered and overthrown the Kingdom of the Stormlands. Instead of granting the dominion of Storm's End to House Blackgard, who descended from House Durrandon - the previous Storm Kings - from the male line, he instead gave the Stormlands to House Baratheon, who are not blood-related to the Durrandons and who many believe do not hold official claim to the castle. This ended with Andyron Blackgard, the firstborn son of Andaren Blackgard, to enter in open rebellion to the Baratheons and the Targaryens.
Instead of traditional warfare, the Blackgards utilized guerrilla warfare, making it harder for Aegon's troops to fight Andyron and his forces head-on at multiple places all at once. This culminated in the exhaustion and eventual stalemate of House Targaryen, and eventually Aenys the First, Aegon's son and heir, obliged to Andyron's pleas for independence, and the constitutional region of the Borderlands was founded.