Nuclear umbrella

Nuclear umbrella refers to a guarantee by a nuclear weapons state to defend a non-nuclear allied state. It is usually used for the security alliances of the United States with Japan,South Korea, the North Atlantic Treaty Organization (much of Europe, Turkey, Canada), and Australia, originating with the Cold War with 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.

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. 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.

NATO involved others of the five official 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.

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