Poland 1939: When German Soldiers Became War Criminals: September 1st, 1939. Nazi Germany invades Poland. The campaign is fast, cruel and ruthless. In these circumstances, how is it that ordinary German soldiers suddenly became vicious killers, terrorizing the local population? Did everyone turn into something worse than wild animals? The true story of the first World War II offensive that marks in the history of infamy the beginning of a carnage and a historical tragedy.