Betar (בית"ר) originally was founded as a Revisionist Zionist Jewish Youth Movement and later Jewish Resistance Organization in Latvia by Ze'ev Jabotinsky. Betar lobbied for the establishment of an independent Jewish state in the former British Mandate territory of Palestine (Eretz Israel), eventually among many other things resulting in the founding of The Jewish Legion, of which units were fighting on the side of the British against Ottoman rule.
Probably the best well-known Jewish fighting unit was called The Zion Mule Corps, led by Yosef Trumpeldor of whom the name Betar is derived: 'Brit haNo'ar ha'Ivri al shem Yosef Trumpeldor' (Jewish Youth Movement in remembrance of Yosef Trumpeldor). Betar also was the name of a fortress near Bethlehem, where Shimon Bar Kochba fell during the 'Bar Kochba-rebellion' against the Romans in the second century áfter.
The Jewish Legion inspired many paramilitary organizations who fought for independent existential rights of the Jewish people, both in and outside Eretz Israel: HeHalutz, Hagana, Palmach, Irgun Zvai Leumi (Hagana Bet), Brit HaBirionim, NILI, Lehi, and the ZZW. Historians agree to the fact these militant organizations were the forerunner of the present-day Israel Defense Forces (IDF).
In the 1920's, 1930's and 1940's until the establishment of the independent State of Israel in 1948, Betar and prominently the Irgun Zvai Leumi collaborated intensively to organize and develop illegal immigration of European Jewry to the British Mandate Territory of 'Palestine.' However, Trumpeldor and Jabotinsky weren't physically present in Eretz Israel any longer: Trumpeldor was killed during the battle of Tel Haï in 1920, and Jabotinsky got banned for openly resisting against British occupation.
In 1940, the year of Jabotinsky's passing, estimated is the amount of Betar-members was exceeding 90.000, a tenfold of a decade before. Apart from Eretz Israel, there were active branches in Eastern-Europe (The Baltic States), Central Europe, France, England, but also for example in Harbin, China. After the second world war, on the mainland of Europe Betar as a youth movement -exceptions assured- failed to rise once more because of the aftermath of the holocaust and communist-rule in Eastern-Europe. Revisionist Zionism and common Zionism fused once more into one core Zionist movement. The paramilitary organizations which came forth out of Betar went up into the IDF. Politically, the Herut (Freedom) party (a few decades later renamed Likud) came into existence.
Famous politicians like prime-ministers of Israel Menachem Begin, Yitzhak Shamir and Ehud Olmert, but also Eitan Livni, and for example Moshe Arens were part of Betar during the British rule in Eretz Israel. Avraham Stern, founder and leader of the Stern Gang (Lehi), was part of Betar. The Prime-Minister of Israel Benjamin Netanyahu, his predecessor Olmert and also current member of parliament Tzipi Livni were part of Betar's youth movement.