Soukou Kihei Votoms: Pailsen Files

Both are real-robot military mecha with a dark tone, and both also explore the concept of a Shinigami. Usually translated as Death God, or Grim Reaper, a Shinigami is a spirit that is drawn to death and also draws more mortals to their own deaths, usually violently. Naturally an entity like this would be right at home on a battlefield, and both these series play on that theme. Although, IGLOO does take the concept a bit more literally.  

Both OVA's are essentially side stories to the original tv series showing the frontline of the war. TheIr also both slightly more realistic than the TV series and focus on more grunt soldiers than powerfull Newtypes or Perfect Soldiers.