To ease public misgivings about his administration's nuclear ambitions, Satō introduced the Three Non-Nuclear Principles to the Diet in 1967. Retroactively, to lessen the principles' restriction on Japanese military defense options, in 1968 Satō broadened the principles into the "Four Pillars Nuclear Policy" of (1) promoting the use of nuclear power for peaceful purposes, (2) global nuclear disarmament, (3) reliance on the U.S. nuclear deterrent for protection from nuclear attack, and (4) the Three Principles. In particular, the fourth pillar called for adherence to the principles "under the circumstances where Japan's national security is guaranteed by the other three policies", implying that a change of circumstances might allow Japan to develop a nuclear program. Amid anxiety over U.S. involvement in the Vietnam War, aggression between North and South Korea, and tense Cross-Strait relations, this stipulation served to reassure the Diet that the nuclear option would still be considered if any of the conflicts escalated to threaten Japanese national security.