Complex sentence: Because of his nationalist views, he was considered, among others, one of those who were responsible for the anti-Semitic riots in Częstochowa in 1919 in which the soldiers of the Blue Army took part.
Split into simpler sentences: Because of his nationalist views, he was considered, among others, one of those who were responsible for the anti-Semitic riots in Częstochowa in 1919. In which the soldiers of the Blue Army took part.