During the invasion of Poland, which started World War II, on 10 September 1939 German forces reached Ostrołęka. The Germans immediately carried out searches of Polish offices and organizations. Under German occupation the town was annexed directly to Germany and in 1940 it was renamed "Scharfenwiese" to remove traces of Polish origin. The Germans expelled its Jewish minority, which was later mostly murdered in the Treblinka extermination camp.