The Flight of KAL 007

On September 1, 1983, a Korean Airlines jet en route from New York to Seoul strayed off course and over a military-sensitive area of the Soviet Union. The jet refused to respond to warnings by a Soviet fighter jet. The pilot of the fighter was ordered to shoot it down. He did. The plane went down into the Sea of Japan. All 269 people on board were killed. How did this disaster occur? What caused the Soviets to down the commercial flight?