Complex sentence: After negotiations collapsed Petro Doroshenko and his men crossed into the Left-bank Ukraine supporting and uprising against Ivan Briukhovetsky in 1668 and following his execution proposed Demian Mnohohrishny as new Hetman of eastern Ukraine.
Split into simpler sentences: After negotiations collapsed Petro Doroshenko and his men crossed into the Left-bank Ukraine supporting and uprising against Ivan Briukhovetsky in 1668. Following his execution they proposed Demian Mnohohrishny as new Hetman of eastern Ukraine.