Complex sentence: He claimed that this action was undegone in order to fight the Ukrainian Insurgent Army, and his particualr claim was described as a "blatant" mistake by Yale historian and specialist Timothy Snyder.
Split into simpler sentences: He claimed that this action was undegone in order to fight the Ukrainian Insurgent Army. Poliszczuk's specific argument was described as a "blatant" mistake by Yale historian and specialist Timothy Snyder.