From Halopedia, the Halo wiki
Nadja Mielke was the leader of the Frieden movement in the mid-22nd century. In November of 2163, she was assassinated on her way to a command center in the Europan city of Pelagon. The Koslovics were clearly responsible. Her murder enraged the Friedens, and in December they retaliated with a large-scale campaign against the Argyre Planitia on Mars with nearly all the military assets at their disposal.