This wave of immigration began as a pioneering one, but with the onset of racial persecution in Nazi Germany attained the character of a mass migration between 1933 and 1939, with at least 55,000 Jews from Central Europe immigrating to Palestine or residing there as semi-permanent residents. The 1936-1939 Arab riots in Mandatory Palestine weakened the immigration wave, but during the years 1938-1939 thousands of Jewish immigrants arrived, some of them illegally. The British White Paper of 1939 severely curtailed Jewish immigration. The onset of World War II a few months later also inhibited immigration to Mandatory Palestine.