Single Integrated Operations Plan. The name for the United States' plan for nuclear war from the 1950s through the 1960s. Initially, the SIOP embodied massive retaliation, as there were no provisions for lower levels of response; triggering the SIOP meant, in the words of WOPR/Joshua, global thermonuclear war. However, in the face of criticisms that this was detrimental to deterrence (because no-one would believe the SIOP would be triggered for smaller offenses like, say, the invasion of Iran by the Soviet Union in a move for the Persian Gulf oil reserves) the SIOP was broken up into a variety of smaller 'component' missions, which could be executed individually. This was known as the doctrine of flexible response.