SUUJI DE SUKUU! JYAKUSHOU KOKKA (MANGA) - RAW

Description

Soara, the princess of the small country Favell, was troubled every day how to protect his country, which has a low power.
His father fell ill, and his vassals emphasized only the honor of battle and no one had a concrete plan, and tensions with neighboring countries were rising.
This will destroy the country you love ....
However, at that time, a young man in front of her appeared.
The theory and thought of numbers he deals with was the power to handle the country that Soara sought ...!

No magic, no allies! Learn how to win the war with umbers.

No real need to make my personal summary since this is enough clear.