Ikusa Otome Valkyrie "Anata ni Subete o Sasagemasu"

In the High-Fantasy world of Midgard, the Humans, with the direct support of the Gods, are fighting a long and ferocious war for the control of their land and freedom against the Demons from the netherworld (...).
The main character is Duke, an Human half-blood commander in the demonic Knights of Darkness. A cynical but very capable leader, he's nevertheless openly despised by all other Demons, both for his ancestry and his "soft" practices (like, for example, to forbid his own troops the traditional butchering of captured enemy children). But, ironically, it's exactly Duke's unorthodox approach to strategy that could give the final victory to the Demons...

[Edited from a forum post by Baldo]