In September 1939, at the dawn of what would otherwise have been the peak decade of Olga Sławska's career, the invasion of Poland from the west by Nazi Germany and, two weeks later, from the east and south by Soviet Russia marked the start of the Second World War and put an end to her professional ambitions for the foreseeable future. She also faced a second personal tragedy at this time. She had recently become engaged to be married to the wealthy industrialist-politician . Rejecting the opportunity to go abroad, Tadeusz Karszo-Siedlewski remained in Warsaw following the German and Soviet invasions and was killed there in a German air-raid on 24 September 1939. A couple of weeks before he was killed Karszo-Siedlewski drew up a new will in which he left a substantial amount of property to his finacée. She, therefore, found herself the owner of , a substantial brewery in Lublin, and of "Mon Plaisir", a comfortable villa in the then fashionable suburb on the edge of Warsaw known at that time as Błota. It is not clear how much practical benefit she received from her new property, however, since her late fiancé's will was only finalised in 1946, by means of a court settlement with the Karszo-Siedlewski family, shortly after which the brewery was taken over by the new Polish government.