The United Nations responded by establishing its first peacekeeping force, (UNEF). It was stationed between Egypt and Israel and for the next decade it maintained peace and stopped the fedayeen incursions into Israel. In 1959, Ben-Gurion learned from West German officials of reports that the notorious Nazi war criminal, Adolf Eichmann, was likely living in hiding in Argentina. In response, Ben-Gurion ordered the Israel foreign intelligence service, the Mossad, to capture the international fugitive alive for trial in Israel. In 1960, the mission was accomplished and Eichmann was tried and convicted in an internationally publicized trial for various offenses including crimes against humanity, and was subsequently executed in 1962. Ben-Gurion is said to have been "nearly obsessed" with Israel obtaining nuclear weapons, feeling that a nuclear arsenal was the only way to counter the Arabs' superiority in numbers, space, and financial resources, and that it was the only sure guarantee of Israel's survival and the prevention of another Holocaust. During his final months as premier Ben-Gurion was engaged in a, now declassified diplomatic standoff with the United States. Ben-Gurion stepped down as prime minister for personal reasons in 1963, and chose Levi Eshkol as his successor. A year later a rivalry developed between the two on the issue of the Lavon Affair, a failed 1954 Israeli covert operation in Egypt. Ben-Gurion had insisted that the operation be properly investigated, while Eshkol refused. Ben-Gurion subsequently broke with Mapai in June 1965 and formed a new party, Rafi, while Mapai merged with Ahdut HaAvoda to form Alignment, with Eshkol as its head. Alignment defeated Rafi in the November 1965 election, establishing Eshkol as the country's leader. In May 1967, Egypt began massing forces in the Sinai Peninsula after expelling UN peacekeepers and closed the Straits of Tiran to Israeli shipping. This, together with the actions of other Arab states, caused Israel to begin preparing for war. The situation lasted until the outbreak of the Six-Day War on 5 June. In Jerusalem, there were calls for a national unity government or an emergency government. During this period, Ben-Gurion met with his old rival Menachem Begin in Sde Boker. Begin asked Ben-Gurion to join Eshkol's national unity government. Although Eshkol's Mapai party initially opposed the widening of its government, it eventually changed its mind.