Under a public policy guidance, strategic weapons and projects are researched and developed entirely by civilian scientists and engineers, who also develop a wide range of delivery systems. On military policy issues, Pakistan issues directives towards "first use" and maintains that its program is based on nuclear deterrence, to peacefully discourage attack by India and other countries with large conventional-force advantages over Pakistan. According to United States military sources, Pakistan has achieved survivability in a possible nuclear conflict through second strike capability. Since the early 1990s, Pakistan's nuclear strategists have emphasised attaining "second strike" capability in spite of their "first use" policy. Statements and physical actions by Pakistan have cited the survivability through a second strike, forming a naval-based command and control system to serve as "the custodian of the nation's second-strike capability."