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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[citation needed]. Other Arab states mobilized their forces. Israel reiterated that these actions were a casus belli. On 5 June 1967, Israel launched a pre-emptive strike against Egypt. Jordan, Syria and Iraq responded and attacked Israel. In a Six-Day War, Israel defeated Jordan and captured the West Bank, defeated Egypt and captured the Gaza Strip and Sinai Peninsula, and defeated Syria and captured the Golan Heights. Jerusalem's boundaries were enlarged, incorporating East Jerusalem, and the 1949 Green Line became the administrative boundary between Israel and the occupied territories.

When did Egypt mass it's army near the border with Israel?
1967