The Second Aliyah (1904–14), began after the Kishinev pogrom; some 40,000 Jews settled in Palestine, although nearly half of them left eventually. Both the first and second waves of migrants were mainly Orthodox Jews, although the Second Aliyah included socialist groups who established the kibbutz movement. During World War I, British Foreign Secretary Arthur Balfour sent the Balfour Declaration of 1917 to Baron Rothschild (Walter Rothschild, 2nd Baron Rothschild), a leader of the British Jewish community, that stated that Britain intended for the creation of a Jewish "national home" within the Palestinian Mandate.