Complex sentence: Other Chechens also were trained by the GRU in warfare, many of these Chechens who fought for the Russians in Abkhazia against Georgia had fought for Azerbaijan against Armenia in the Nagorno-Karabakh war.
Split into simpler sentences: Other Chechens also were trained by the GRU in warfare. Many of these Chechens who fought for the Russians in Abkhazia against Georgia had fought for Azerbaijan against Armenia in the Nagorno-Karabakh war.