"Syria's attacks on Israeli kibbutzim from the Golan Heights finally provoked a retaliatory strike on April 7, 1967. During the attack, Israeli planes shot down six Syrian fighter planes — MiGs supplied by the Soviet Union. Shortly thereafter, the Soviets — who had been providing military and economic assistance to both Syria and Egypt — gave Damascus false information alleging a massive Israeli military buildup in preparation for an attack. Despite Israeli denials, Syria decided to invoke its defense treaty with Egypt and asked Nasser to come to its aid."

http://www.jewishvirtuallibrary.org/jsource/History/67_War.html