Ouzoku

In a world where the victor gets to be king (王, "Ou"), and loser goes down in history as bandits (賊, "Zoku"), the kingdom of Visto started a large scale military expansion to dominate the continent.

The main character, Zin, was a famous strategist defending his nation against the invasion and had much success leading his "Ouzoku" unit in a guerrilla campaign behind enemy lines, but due to traitors back home, his nation was eventually conquered. Now, the kingdom of Elt seeks his expertise to help them defend against the Visto army. To Zin, this is a great opportunity for revenge against Visto, and well as a chance to get a kingdom's ransom worth of... women!

[Based on Wikia]