Kage no Kyuutei Majutsushi: Munou dato Omowareteita Otoko, Jitsu wa Saikyou no Gunshi Datta

Leon is the kingdom's most worthless Court Magician. One day, his repeated absences are punished and he is sent to the battlefield, but with the quick wits he was hiding he saved the Third Princess Sicily? Then, along with the reformer princess, and steeling his own resolve, Leon becomes a specialized officer determined to control events from the shadows.

(Source: Overlap, translated)