The signal event of this aliyah wave was the Nazi accession to power in Germany (1933). Persecution and the Jews' worsening situation caused aliyah from Germany to increase, and aliyah from Eastern Europe to resume. Many of the immigrants from Germany were professionals; their impact was to be felt in many fields of endeavor. Within a four-year period (1933-1936), 174,000 Jews settled in the country. The towns flourished as new industrial enterprises were founded and construction of the Haifa port and the oil refineries was completed. Throughout the country, "stockade and tower" settlements were established. During this period in 1929 and again in 1936-39 violent Arab attacks on the Jewish population took place, called "disturbances" by the British. The British government imposed restrictions on immigration, resulting in Aliyah Bet — clandestine, illegal immigration.
By 1940, nearly 250,000 Jews had arrived during the Fifth Aliyah (20,000 of them left later) and the yishuv's population reached 450,000. From this time on, the practice of "numbering" the waves of immigration was discontinued which is not to say that aliyah had exhausted itself.