The shield of Westeros was a grand coalition of Kings formed against the invasion of the Targaryen king Aegon I. This coalition included The Rock led by King Loran Lannister, The Reach led by House Gardener, The stormlands led by House Durrandon, and Dorne led by House Martell. This coalition is considered the biggest and rarest coalition formed in the recent history of Westeros, since the kings were mostly independent before the invasion of Westeros. The coalitions started their first attack in an attempt to defend the riverlands from the invasion. A grand battle held was held in The Antlers between 10.000 dragonstone troops of Aegon I + his Dragon and the 55.000 Coalition troops. The Shield of Westeros ultimately won the battle, but took grievous losses, including the lifes of King Mern IX of the Reach and King Argilac I of the Stormlands. They remained in the riverlands for a couple of months. Afterwards the Reach tried the invade Dragonstone but failed when Aegon I managed to break the siege just before it had ended. King Aegon I took his fleet to Sunspear which resulted in an immediate capture of the castle. King Aegon I forced the Shield of Westeros to stand down, threatening to kill all the members of house Martell. With the loss of Dorne and the Martells the Coalition decided to stand down and bend the knee. The coalition ultimately held out for nearly 3 years.
"FIGHT ON, BROTHERS! FOR WESTEROS!" - King Mern XI of the Reach
King Loran IV bent the knee after Aegon I accepted his offer to join houses.
Queen Argella, daughter of Argilac, died in battle 5 years after the end of the coalition. Her daughter Lia was then forced to bend the knee.