Zionism is a political movement, one that calls for creation of a specifically Jewish state. When the movement began in the late 1880s, anti-Semitism was a powerful and growing force in Russia and Europe. Most Jews at that time believed that the best way to stop anti-Semitism was either through some kind of assimilation, or through alliances with other political movements. But a small number of Jews believed that anti-Semitism could never be overcome, and that the only way for Jews to be safe would be for them to leave their home countries and establish a Jewish state somewhere.
Early Zionist leaders believed that a Jewish state could be established anywhere (Uganda, Argentine, and Turkey were both considered at different times); it was a thoroughly secular movement. But the founder of the modern Zionist movement, Theodore Herzl, recognized that linking Zionism to Palestine would gain wider support among more Jews. Herzl also believed that a Jewish state could only be created with the support of a colonial sponsor, and he travelled the imperial capitals of the world seeking a patron.
Many Jews opposed Zionism. The ultra-orthodox Jews in Palestine believed that only God could deliver a state to the Jewish people, and that a human-based effort was against God's will. Many Jews facing anti-Semitic attacks rejected Zionism's call for them to leave their homelands, seeing that position as mimicking the demand of the anti-Semites themselves.
The Zionist movement won strong support from the British when London took control of Palestine with the defeat of the Ottoman Empire. In 1917 the Balfour Declaration stated that "His Majesty's Government views with favour the establishment in Palestine of a national home for the Jewish people, ... it being clearly understood that nothing shall be done which may prejudice the civil and religious rights of existing non-Jewish communities in Palestine." In the stroke of a pen the vast majority of the population of Palestine was reduced to the "non-Jewish community.”
Zionism gradually gained more adherents, though slowly. It was only in the 1930s and '40s, as German and Polish and other European Jews desperately sought to escape Hitler and their first choice countries of refuge, the U.S. and Britain, denied them entry, that Zionism and the call to create a Jewish state in Palestine, became a more popular view among Jews. After World War II, with Holocaust survivors filling Displaced Person camps across Europe, Zionism became the majority position.
The Zionist slogan was that Palestine was "a land without a people for a people without a land." Certainly the second part was true—the European Jews who had survived the Holocaust had lost everything—their homes, their families, their countries, their land. Turned away from the U.S. because of anti-Semitism, and encouraged to go to Palestine instead, it was not surprising that thousands flocked to join Jewish communities there. But the first part of the slogan hid the reality—for Palestine was not a land without a people. It's indigenous people had been there all along.
With the creation of Israel, the organizations of the Zionist movement such as the Jewish Agency became adjuncts of the state apparatus, focusing on recruiting and settling Jews from all over the world in Israel.