Strike Witches: Maidens in the Sky Vol. 1 (manga)

The titular Strike Witches, a group of young women with high magical potential, have been gathered together from various nations in a task force to fight against the enigmatic Neuroi. This puzzling enemy force has appeared frequently and without warning in many areas across the world. The weapons of the Neuroi mostly take on forms similar to propeller aircraft but their most troubling tactic is the spreading of a corrosive miasma. Not only do normal humans have no defense against this miasma, but the remnants of the land affected by it are processed by the Neuroi into new weapons, crumbling huge sections of former nations into the sea. As the miasma seems to be unable to spread across large bodies of water, humanity has designated such areas as their main lines of defense.