It's about a war between Koukoku (The Imperial country) and Teikoku (The Empire).

The main charactor Shinjou Naoe, First Lieutenant of the Koukoku army, is thrown into a long desperate war. The sudden invasion by Teikoku East division crushed the Koukoku North Defense Forces. Shinjou and his 600 men battalion was ordered to battle in the rear and delay the advance of Teikoku 40,000 men army.

(Source: ANN)