Since 1964, Arab countries, concerned over Israeli plans to divert waters of the Jordan River into the coastal plain, had been trying to divert the headwaters to deprive Israel of water resources, provoking tensions between Israel on the one hand, and Syria and Lebanon on the other. Arab nationalists led by Egyptian President Gamal Abdel Nasser refused to recognize Israel and called for its destruction. By 1966, Israeli-Arab relations had deteriorated to the point of actual battles taking place between Israeli and Arab forces. In May 1967, Egypt massed its army near the border with Israel, expelled UN peacekeepers, stationed in the Sinai Peninsula since 1957, and blocked Israel's access to the Red Sea. Other Arab states mobilized their forces. Israel reiterated that these actions were a ''casus belli'' and, on 5 June, launched a pre-emptive strike against Egypt. Jordan, Syria and Iraq responded and attacked Israel. In a Six-Day War, Israel captured and occupied the West Bank from Jordan, the Gaza Strip and Sinai Peninsula from Egypt, and the Golan Heights from Syria. Jerusalem's boundaries were enlarged, incorporating East Jerusalem, and the 1949 Green Line became the administrative boundary between Israel and the occupied territories.