Shinryuu Teikoku no Dragon Tamer

Raid, a dragon tamer with exceptional skills, had single-handedly managed the dragons, the empire's main fighting force. However, one day, the emperor fires him.
It is then that Luna, the dragon princess whom Raid had previously saved, appears before him. Following her suggestion, Raid sets off for the dragon kingdom, a hidden land…

(Source: Hifumi Shobo, translated)