The Yishuv wanted to help their fellow Jews, who were being murdered by the Nazis in Europe. Many Jews from Europe were prevented from fleeing to Mandatory Palestine by strict immigration quotas established by the white papers. The Jewish Agency organized illegal immigration from 1939 through 1942 with the help of the Haganah. Those who arrived illegally to Israel during this time were part of the Aliyah Bet. This was a dangerous operation, for these illegal immigrants arrived by boat and had to be careful not to be caught by the British or Nazis. Many of these ships sank or were caught, such as the "Patria", "Struma" and . Compared with the number of attempts, few ships actually arrived successfully to Mandatory Palestine, but tens of thousands of Jews were saved by the illegal immigration.