or at least retain second-strike capability. Although the Cold War ended in the early 1990s, the MAD doctrine continues to be applied. Proponents of MAD as part of the US and USSR strategic doctrine believed that nuclear war could best be prevented if neither side could expect to survive a full-scale nuclear exchange as a functioning state. Since the credibility of the threat is critical to such assurance, each side had to invest substantial capital in their nuclear arsenals even if they were not intended for use. In addition, neither side could be expected or allowed to adequately defend itself against the other's nuclear missiles. This led both to the hardening and diversification of nuclear delivery systems (such as nuclear missile silos, ballistic missile submarines, and nuclear bombers kept at fail-safe points) and to the Anti-Ballistic Missile Treaty. This MAD scenario is often referred to as rational nuclear deterrence. Theory of mutually assured destruction When the possibility of nuclear warfare between the United States and Soviet Union started to become a reality, theorists began to think that mutual assured destruction would be sufficient to deter the other side from launching a nuclear weapon. Kenneth Waltz, an American political scientist, believed that nuclear forces were in fact useful, but even more useful in the fact that they deterred other nuclear threats from using them, based on mutually assured destruction. The theory of mutually assured destruction being a safe way to deter continued even farther with the thought that nuclear weapons intended on being used for the winning of a war, were impractical, and even considered too dangerous and risky. Even with the Cold War ending in 1991, about 30 years ago, deterrence from mutually assured destruction is still said to be the safest course to avoid nuclear warfare. History Pre-1945 The concept of MAD had been discussed in the literature for nearly a century before the invention of nuclear weapons. One of the earliest references comes from the English author Wilkie Collins, writing at the time of the Franco-Prussian War in 1870: "I begin to believe in only one civilizing influence—the discovery one of these days of a destructive agent so terrible that War shall mean annihilation and men's fears will force them to keep the peace." The concept was also described in 1863 by Jules Verne in his novel Paris in the Twentieth Century, though it was not published until