At the same time, the Irgun also established itself in Europe. The Irgun built underground cells that participated in organizing migration to Palestine. The cells were made up almost entirely of Betar members, and their primary activity was military training in preparation for emigration to Palestine. Ties formed with the Polish authorities brought about courses in which Irgun commanders were trained by Polish officers in advanced military issues such as guerrilla warfare, tactics and laying land mines. Avraham (Yair) Stern was notable among the cell organizers in Europe. In 1937 the Polish authorities began to deliver large amounts of weapons to the underground. According to Irgun activists Poland supplied the organization with 25,000 rifles, and additional material and weapons, by summer 1939 the Warsaw warehouses of Irgun held 5,000 rifles and 1,000 machine guns. The training and support by Poland would allow the organization to mobilize 30,000-40,000 men The transfer of handguns, rifles, explosives and ammunition stopped with the outbreak of World War II. Another field in which the Irgun operated was the training of pilots, so they could serve in the Air Force in the future war for independence, in the flight school in Lod.