18+ Suuji de Sukuu! Jakushou Kokka

With tensions between the neighboring country rising signaling the presence of a war soon to begin, Soalla, the princess of a small country, worried whether her own country would be able to defend itself with its weak national power. The king was bedridden and the retainers who were her last rays of hope had nothing but the antiquated motto of battle honor in mind. Not one person had a concrete proposal to protect the country, leaving Soalla to wonder if this was where Favell would fall... However, it was then that Naoki appeared before her. A "magician," that left his name in history who possessed the power she desired to save the country.

(Source: Kadokawa, translated)