Alethkar is a highly militarized nation. Their religion believe that the only point of living is to become soldiers in the Tranquiline Halls. (Technically to achieve their Calling, but the Alethi believed that the best of all Callings was a soldier, so same thing.) That being the case, they literally itched for an excuse to go to war - and they had one in the Sunmaker, the king who originally united all ten princes of Alethela and went on to conquer large swaths of lands, including what eventually became Jah Keved. There was talk of going to war because that was what Alethkar did, and Jah Keved was an easy target. Speaking the same language and sharing the same religion is very enticing from a conqueror's perspective - they're easier to govern and integrate into your own empire that way.

Alethkar is a highly militarized nation. Their religion believes that the only point of living is to become soldiers in the Tranquiline Halls. (Technically to achieve their Calling, but the Alethi believed that the best of all Callings was a soldier, so same thing.) That being the case, they literally itched for an excuse to go to war - and they had one in the Sunmaker, the king who originally united all ten princes of Alethela and went on to conquer large swaths of lands, including what eventually became Jah Keved. There was talk of going to war because that was what Alethkar did, and Jah Keved was an easy target. Speaking the same language and sharing the same religion is very enticing from a conqueror's perspective - they're easier to govern and integrate into your own empire that way.