Illegal immigration to Palestine began at the time of the Ottoman Empire. During the early part of the British mandate, there was no need for illegal immigration, as the British policy was to allow nearly as many Jews to settle in Palestine as wished to do so. With the rise of Hitler and the growing flood of Jewish emigration, the Revisionists organized small groups of illegal immigrants. In 1938, the Histadrut organized the Mossad L'Aliyah Bet, as its central clearing house for illegal immigration. In the years before the outbreak of World War II, 41 ships arrived containing approximately 16,000 illegal immigrants. During World War II, there were continued attempts to organize illegal immigration. Between 1939 and 1944, 26 vessels sailed, primarily from Bulgaria and Romania. Of this 26, five were sunk en route. Altogether, 16,500 individuals sailed toward Palestine during the war years. In addition, 3,740 immigrants were brought into Palestine overland. At the end of World War II there were over 600,000 displaced Jews in Europe. Between 1945 and 1948, 69,000 of these Jews were brought from Europe by ship. Although the majority of them were intercepted by the British ships and sent to Cyprus for internment, the act of the illegal immigration underscored the importance of a Jewish State for the Jewish refugees.
Most of the illegal immigrants ended up in British detention camps in Cyprus.