Complex sentence: The troops, which were mainly responsible for the war crime, were led by Bronislav Kaminski, a Russian who commanded the collaborating S.S. Sturmbrigade R.O.N.A., the so-called "Russian National Liberation Army" (RONA).
Split into simpler sentences: The troops, which were mainly responsible for the war crime, were led by Bronislav Kaminski. He was a Russian who commanded the collaborating S.S. Sturmbrigade R.O.N.A., the so-called "Russian National Liberation Army" (RONA).