The Beginning War is somewhat of a misnomer, as it happened quite a while after the creation of the Realm. It was a struggle for supremacy of the land, primarily fought between the gods and the primordials that had shaped much of the landscape, though other interests served on either side or played their own hands.
The war was long and the cost was great. The Realm suffered greatly and much of the land was reshaped. Entire parts of the land sunk beneath the waves and other parts were forced into new mountain ranges. In the end, the gods defeated the primordials and sealed them away in extraplanar prisons. The devastation of the war lead the gods to make the decision to distance themselves from the Realm.