If you're looking for manga similar to SiM: The Rumbling, you might like these titles.
Yamato is a small country at war with the military might of Mars. In the face of this overwhelming superiority, Yamato's defeat seems inevitable. Even on the island of Gallé in the South Pacific Ocean, where fierce battles are also being fought, Yamato's army is in a state of collapse. Sergeant Tatsumi fights for the survival of his regiment, which is in a desperate situation. One day, just when all hope seems to be gone, he sees an "object" named Kitetsu drop from a transport plane...
In her past life, Asura was a soldier of fortune who lived for the battle above all else. So when she reincarnates into a world with magic, she gladly takes up the mercenary’s path for a second time. Drawing upon her battle-hardened experience, she forms the mercenary group Moon Blossom and pioneers the concept of the soldier-mage: magic users who also fight with the skills and training of military soldiers. Roaming the battlefields of this new world, Moon Blossom—led by Asura and her unrivaled schemes, ruthlessness, and tactical ingenuity—earns a fearsome reputation. Thus the curtain rises on the dark fantasy epic of a young girl who is spoken of in fearful whispers as “The Silver Demon”!
The nations of Oris and Gondnea are at war with each other. Oris’s oppressive military force wants nothing more than to track down something called Skanda – a secret safely kept with the unknowing Mio, a little girl with a hyper-sensitivity to sound. One day, Mio witnesses a Gondean helicopter crash and meets one of its passengers, Rico; but Rico’s memory only lasts for three minutes at a time, and she doesn’t remember what she’s doing in Oris. Together, the two must stay one step ahead of the forces of Oris if they are to stay alive...