Studies began in 1972–1973 to provide more flexibility for the use of American nuclear weapons. In January 1974 President Richard M. Nixon approved NSDM-242, intended to add more "limited employment options" to help manage escalation, to SIOP-63. The related Nuclear Weapons Employment Policy (NUWEP) of April 1974 provided targets to achieve various goals; for example, the document stated that the United States nuclear forces must possess the ability to destroy 70% of the industrial capacity the Soviet Union needed to recover after a war. These documents formed the basis of SIOP-5 (January 1976), sometimes called the Schlesinger Doctrine after Secretary of Defense James Schlesinger. The ever-expanding target lists were split into classes of targets, with a wider range of plans matching strikes to political intentions from counterforce to countervalue, or any mix/withhold strategy to control escalation. Schlesinger described the doctrine as having three main aspects: