He became (1938) leader of a Zionist youth movement in Poland, where he also earned a law degree. Begin went to Palestine in 1942; there, he headed the Irgun, a militant organization that fought against the British Mandate authorities. After 1949 he sat in the Knesset, where he led the opposition to the Labor party. In May, 1977, Begin's right-wing Likud party defeated Labor for the first time, and Begin became prime minister. He shared the 1978 Nobel Peace Prize with Egyptian President Anwar al-Sadat as a result of the Camp David Accords. In 1982, Begin authorized a massive invasion of Lebanon in order to destroy military bases of the Palestine Liberation Organization (see Arab-Israeli Wars). The war caused intense domestic and international pressure and failed to achieve Israel's principal aims. Begin resigned from office in 1983.