The Tokyo Great Decisive Battle, also known as Great Tokyo War. This is an important historical event that happened 10 years before the anime starts. The exact name of the battle is unknown because of differing translations. The battle is also called the Great Tokyo War or other alternatives.
The Battle happened 10 years before the start of the anime and caused Japan to split into 10 Independent Nations. The Magnitude of the war is unknown.
Japan was one whole country.