A "nuclear umbrella" is a guarantee by a nuclear-weapon state to defend a non-nuclear allied state. The context is usually the security alliances of the United States with Australia, Japan, South Korea, the North Atlantic Treaty Organization (much of Europe, Turkey and Canada) and the Compact of Free Association (the Federated States of Micronesia, the Marshall Islands, and Palau). Those alliances were formed because of the Cold War and the Soviet Union. For some countries, it was an alternative to acquiring nuclear weapons themselves; other alternatives include regional nuclear-weapon-free zones or nuclear sharing. Nuclear umbrellas are also referred to as "extended deterrence" under which a nuclear-weapon state extends its nuclear deterrent to cover threats against its non-nuclear-weapon state allies.
United States-provided The United States has promised its role as a "nuclear umbrella" for numerous non-nuclear allied states, even as early as the Cold War. The US now has security alliances of this nature with around 30 countries, many within NATO itself. The country also has notable arrangements of this type with South Korea and Australia. The US understood the power of deterrence with nuclear weapons early on, beginning with the concept of massive retaliation during the Eisenhower administration. As the USSR and other countries became nuclear powers as well, however, the risk of any nuclear exchange became more clear. This, in part, motivated the US to adopt the new strategy of deterrence, in which they would have more control over the situation, while still maintaining the ability to intervene in conflicts, a nuclear umbrella. The US provides protection and deterrence for various countries under its umbrella, and in turn, the countries do not pursue nuclear weapons programs themselves. More recently, however, concerns have been raised about the diminishing power of such a threat, due to the rapid increase of nuclear weapons of mass destruction across the globe. Russia in particular has caused concern, having focused their military doctrine on nuclear weapons, as well as continued in the development of their weapons programs. Currently, the United States holds only some "nonstrategic" military weapons in Europe, and these nonstrategic weapons aid in reassuring countries under the umbrella, and emphasizing their role as a deterrent. The strategy of deterrence remains unequivocally important for the country, but many argue that the US will face various new challenges when it comes to the rise of other nuclear powers and weapons of mass destruction.
To NATO NATO was formed early in the Cold War and, from the beginning, assumed American nuclear power as a major component of defense of Western Europe from possible Soviet invasion. NATO Article 5 allows for collective defense to a military attack on any NATO member. Most non-Communist European states joined the alliance, although some (Ireland, Switzerland, Austria, Sweden, Finland) instead maintained an official policy of neutrality. Sweden and Switzerland considered developing their own nuclear weapons but abandoned the idea. Apart from European states, the NATO nuclear umbrella extends to Canada. NATO involved others of the five NPT-designated nuclear weapons states. The United Kingdom and Canada participated in the initial American development of the atomic bomb (Manhattan Project) during World War II, but were afterwards excluded from nuclear weapons secrets by act of the US Congress. Britain launched an independent nuclear weapons program; after Britain successfully developed thermonuclear weapons, the US and UK signed the 1958 US–UK Mutual Defence Agreement sharing American weapons designs, eliminating the need for independent development. Canada has not officially maintained and possessed weapons of mass destruction since 1984 (Canada and weapons of mass destruction) After the end of the Cold War, many Central and Eastern European countries joined NATO.
In Asia
To Japan The Japanese nuclear weapon program was conducted during World War II. Like the German nuclear weapons program, it suffered from an array of problems, and was ultimately unable to progress beyond the laboratory stage. Following the atomic bombings of Hiroshima and Nagasaki, World War II and the deconstruction of the imperial military, Japan came under the US "nuclear umbrella" on the condition that it will not produce nuclear weapons. This was formalized in the Security Treaty Between the United States and Japan, which preceded the current security alliance, the Treaty of Mutual Cooperation and Security between the United States and Japan. Japan and the United States also have a major missile defense accord to mitigate the North Korean nuclear threat, among others and have deployed the Aegis Ballistic Missile Defense System jointly.
To South Korea Following the Korean War, South Korea was welcomed under the US "nuclear umbrella" after signing the ROK-US mutual security treaty on October 1, 1953. This was characteristic of US defense and foreign policy at the time, which championed extended deterrence in an effort to prevent any nuclear conflict. The agreement also aligned with the US nonproliferation objectives, by eliminating the need for South Korea to develop its own nuclear weapons program. In the ROK-US mutual security treaty, the US agreed to deter attacks against South Korea and defend them in the case of attacks, and to deploy troops at the Korean Demilitarized Zone. The US also positioned tactical nuclear weapons on the Korean Peninsula, but these weapons were retracted by President Bush in September 1991. The US nuclear umbrella over South Korea has persisted for almost 70 years. Most agree on the necessity and significance of the US nuclear umbrella and the ROK-US treaty for South Korea, and expect it to hold its place. In 2013, two Northrop B-2 Spirit strategic bombers flew to and from their home airbase in Missouri, dropping inert munitions off South Korea's coast, in a show of force against North Korea and to its allies in South Korea and Japan. In 2023, the USS Kentucky nuclear-armed ballistic missile submarine docked in South Korea, marking the first presence of nuclear weapons in South Korea since the US withdrew its tactical weapons in 1991, and the first US nuclear-armed submarine visit to South Korea since 1981.
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