During World War I, the British government sought Jewish support. In 1917 Weizmann secured from the British government the Balfour Declaration, which promised support for "the establishment in Palestine of a national home for the Jewish people," though nothing was to be done that might "prejudice the civil and religious rights of the existing non-Jewish communities." It did not promise an independent statein Palestine. The League of Nations created a British mandate, with full control over Palestine in 1922.
Zionism grew rapidly in the U.S. after 1900, based largely among Yiddish-speaking recent immigrants from Russia. The Reform Jews, of German background, largely opposed the movement with the main exception of Louis Brandeis, the Supreme Court justice who became a key leader. There were three main groups in 1918: the Zionist Organization of America had 149,000 members, the Mizrachi religious Zionists had 18,000 and the Labor Zionists ( Poalei Zion) had 7,000. The pro-Zionist Yiddish language daily newspapers of the periodthe Yidishes Tageblat, Morgen Zhurnal, the Maccabaean, and Der Togtogether had a combined circulation in 1917 of over 200,000.
The fighting between the Yishuv and the Palestinians, November 1947-14 May 1948, and the war between Israel and invading Arab armies, 15 May 1948-July 1949, represented a total war, as did the Six Day War pf 1967. The life and death of Israel were at stake and required the mobilization of not only the military but also the civilian population, the economy, and the social and political institutions of the Jewish communityas well as mobilizing support from the diaspora in the United States. These crises are a central part of Zionist memories and identity, in combination with memory of the Holocaust, and permanently shaped the Israeli-Arab situation as well as the evolution of the Israeli and Palestinian societies.
In the United States and Europe, until World War II the Reform Jews generally opposed Zionismoften denouncing it because it was the opposite of assimilation and American identity. In the U.S. opposition was centered in the American Council for Judaism. With the establishment of Israel in 1948 the opposition softened, and when the Six Day War in 1967 showed Israel was vulnerable to attack, most previously negative Jews became supportive.