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.
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 Five Eyes treaty sharing American weapons designs, eliminating the need for independent development.
After the end of the Cold War, many Central and Eastern European countries joined NATO, although the original purpose of defense against the Soviet Union was by then obsolete. Some commentators opposed this NATO enlargement as unnecessarily provocative to Russia.
As late as 1970, Australia considered embarking on nuclear weapons development but finally agreed to sign the Nuclear Non-Proliferation Treaty. Since then Australia has been a proponent of nuclear disarmament.
The ANZUS alliance originally included Australia, New Zealand and the U.S., but after New Zealand's nuclear-free zone was proclaimed, the U.S. no longer considered the country to be under its nuclear umbrella.
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 itself won't 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.
Russian nuclear umbrella
The term is far less used for Russian nuclear guarantees, but is seen occasionally.
Missile defense would provide an "umbrella" of another kind against nuclear attack. This is not the conventional usage of "nuclear umbrella", but a rhetorical device promoting active defense over the nuclear deterrence the conventional "nuclear umbrella" depends upon.
- Hans M. Kristensen (1999-07-21). "Japan Under the US Nuclear Umbrella". Nautilus Institute. Archived from the original on 2008-04-22. Retrieved 2007-12-04.
- "The US Nuclear Umbrella Over South Korea". the Nuclear Information Project. 2006-10-23. Retrieved 2007-12-04.
- Nicola Butler, Otfried Nassauer, and Daniel Plesch (1997-02). "Extending the Nuclear Umbrella:Undermining the Nuclear Non-Proliferation Treaty". Berlin Information-center for Transatlantic Security. Retrieved 2007-12-04.
- http://www.theage.com.au/national/when-australia-had-a-bombshell-for-us-20080705-32ai.html The Age, When Australia had a bombshell for U.S.
- Baker Spring (2004-10-03). "Finally, U.S. Gets a Nuclear Umbrella". Heritage Foundation. Retrieved 2007-12-04.