The British government limited Jewish immigration to Mandatory Palestine with quotas, and following the rise of Nazism to power in Germany, illegal immigration to Mandatory Palestine commenced. The illegal immigration was known as "Aliyah Bet" ("secondary immigration"), or "Ha'apalah", and was organized by the Mossad Le'aliyah Bet, as well as by the Irgun. Immigration was done mainly by sea, and to a lesser extent overland through Iraq and Syria. During World War II and the years that followed until independence, "Aliyah Bet" became the main form of Jewish immigration to Mandatory Palestine.