Around 96 AC, the Free Cities of Lys, Myr and Tyrosh formed an alliance to battle the forces of Volantis in The Disputed Lands, soundly defeating them at the Battle of the Borderland.
The new Triarchy made the decision to be ruled by a council of thirty-three magisters rather than a King. The mighty Triarchy felt the need to expand its territory and looked east to the Stepstone Islands after the defeated Volantenes withdrew from the eastern Disputed Lands.
Many times in HotD, the Stepstones are mentioned, although some fans might not know where they are or what they mean. Between eastern Dorne and the Disputed Lands of Essos, there lies a group of rocky, wind-swept islands.
The Triarchy's army and naval force are under the command of Myrish Prince Admiral Craghas Drahar, who is entrusted with seizing control of the Stepstones and purging it of pirates.
Shortly after seizing possession of the Stepstones, the avaricious Craghas Crabfeeder and his Lysene and Tyroshi counterparts started charging shipping ever-increasing tolls until the charges amounted to nothing less than extortion.
The world of House of the Dragon is cruel, and the methods of the Lyseni are among the cruelest.
Lord Corlys Velaryon, one of Westeros' most powerful men, is in charge of a large fleet, but he has discovered that the Crabfeeder's oppressive tolls have put his House's ability to survive economically in peril.
Prince Daemon gathered an army of "landless adventurers and second sons," while the Sea Snake built his fleet, both eager for combat and glory against the Triarchy.
King Viserys provided funds to Prince Daemon's army of "sellswords and cutthroats" to support the Triarchy offensive after learning of the death of the Crabfeeder.
Prince/King Daemon's moment of victory was quickly overshadowed by Dorne's entry into the conflict on the side of the Kingdom of the Three Daughters, which had gathered a new invading force under the direction of a crafty Tyroshi captain by the name of Racallio Ryndoon. The war started up again.