Musket Girls! ~Tensei Sanbou To Senretsu Otome-Tachi~ summary:

After my previous life was cut short, I was reincarnated in a world of early modern-style warfare where line infantry exchanges gunfire. Being a commoner second lieutenant, I was shunned as "Krombertz the Grim Reaper" because of the many deaths of my men in battle. With that, I was assigned to the 6th Special Tasks Brigade, an all-female unit, as a staff officer. The brigade commander, who is up to something, and the carefree non-commissioned officer are women, while the foot soldiers are just girls who are practically amateurs. In this world, if you don't fight, you will be killed. That being the case, I have to do something about this... but how?!