Argues that, to limit nuclear proliferation, the superpowers should seek arms control quantitative limits that permit modernizing their strategic weapon systems (e.g., MIRVing of missiles). Other nations will complain that such superpower "vertical proliferation" entitles them to evade the Non-Proliferation Treaty. Nonetheless, such superpower modernization should on balance deter nuclear spread by making other nuclear programs (e.g., China's) clearly less cost-effective. To be presented to the Conference sponsored by the Norwegian Institute of International Affairs, March 9-13, 1971, at Oslo, on the "Role of Nuclear Weapons in the Politics and Defense Planning of Non-Nuclear Weapon States." 30 pp.