Protector of the Realm

The Protector of the Realm was the third and final title taken by the reigning monarchs of Westeros. However, it was sometimes donned by the in the event that the monarch was too young. Unlike the other two titles, this title was military in nature, granting its wielder dominion over the armies of Westeros. It essentially made all of the Great Houses of Westeros, and their bannermen, bannermen of the royal family.

History
The title was established during Aegon's Conquest and persisted after, claimed by whomever sat the Iron Throne of Westeros.