Between 1919 and 1923, 40,000 Jews, mainly from Eastern Europe arrived in the wake of World War I. The British occupation of Palestine and the establishment of the British Mandate created the conditions for the implementation of the promises contained in the Balfour Declaration of 1917. Many of the Jewish immigrants were ideologically driven pioneers, known as "halutzim", trained in agriculture and capable of establishing self-sustaining economies. In spite of immigration quotas established by the British administration, the Jewish population reached 90,000 by the end of this period. The Jezreel Valley and the Hefer Plain marshes were drained and converted to agricultural use. Additional national institutions arose such as the Histadrut (General Labor Federation); an elected assembly; national council; and the Haganah, the forerunner of the Israel Defense Forces.