Fire Emblem Heroes Book 1, BLADE

From the introduction: Many years ago, the royal families of the Embla and Askr Kingdoms worked together to open and close gates to various worlds. Then, one day, Embla refused to close the gates of the Askr Kingdom and invaded other worlds. Askr went to war against Embla to protect the other worlds. This war waged until the Emblian king died, and his second wife ended his warlike policies. So well-liked was the King of Embla that many people committed suicide upon his death, and the empress wasn’t as well-liked. Then the Emblian King’s daughter, Veronica, restarted the war when she took the throne.