The Six-Day War

The U.S. became involved in the Middle East in the 50’s; after the Iraqi revolution, the U.S. had slightly fluctuating relationships with surrounding countries. In 1962, President Nasser (Egypt) sent troops to fight the Saudi Arabians, who the U.S. backed. In 1963, the U.S. sent aid to Israel in the form of weapons. Johnson withdrew Egyptian aid, allowing the Egyptians to shift further into the Soviet arms. The call for American involvement was also supported by the U.S. statement of 1956 that declared that any Egyptian blockage of the straight of Tiran would be regarded as an act of war. On May 22, 1967, the Egyptians finally made this move. Johnson warns the Israelites not to attack first, but while the Syrians, Egyptians, and Jordanians were moving into position, Israel and the U.S. struck, claiming ultimate victory, albeit shaky.
More interesting and in-depth info can be found here and here.