The Ancient War was a four year war that was between the forces of the Archtitan and Ancieus Uredos' Army of Ancieus in order to defend the balance of the world in the form of Oraclos the Worldholder. The Godgigum Chalice, a God Artifact, was stolen by the Archtitan from the Tyrgarde to generate large amounts of power, enough so he could enter Oraclos' domain. Ancieus' army, made from the large Nordian population, as well as supporters from the very lands he conquered, struck up arms with him in defense of Outhria. The battle was long fought on The World's Edge and Drychwal, and consisted of many factions. The Brotherhood and Confederacy, despite their power and significance, played next to no role within it until the very end of the war in which they were with Ancieus, who proclaimed they all need to defend the world's balance. The war ended with the Archtitan's defeat, and is considered the battle that made Ancieus "the legendary commander".
The Ancient War's end resulted in the two islands, and Krom's Belt (known as The Belt) being concealed in mists, their civilization hidden away. With the Great Turnback, the Ancient War's events returned to reality, forming the Second Ancient War, in A Legend's Legacy: At World's Turn.