A nuclear umbrella is a guarantee by a nuclear weapons state to defend a non-nuclear allied state. The context is usually 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.
France developed a nuclear force de frappe and left the NATO command structure while continuing to be allied with the other Western countries. NATO nuclear Sharing was conceived to prevent further independent proliferation among the western allies. France later rejoined the NATO joint military command on April 4th, 2009.
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 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.
Warsaw Pact (former)
Like NATO, the members of the Warsaw Pact were protected by nuclear weapons of the Soviet Union with the weapons being deployed either in Soviet territory or closer to NATO in territory of the other member states, particularly Poland (see Poland and weapons of mass destruction). Unlike NATO however there was no nuclear sharing and all weapons remained completely under Soviet control. At least one member of the Warsaw Pact, Romania, did consider developing its own arsenal but later abandoned it (see Romania and weapons of mass destruction). Most Eastern European Communist states were part of the Warsaw Pact with the exception of Yugoslavia which became neutral especially after the Tito–Stalin Split, and Albania later left the alliance after the Soviet–Albanian split and aligned itself with the People's Republic of China which had also cut ties with the Soviets in the Sino-Soviet split.
Soviet Allies outside of the Warsaw Pact
It is unclear if and to what extent the Soviet Union's nuclear umbrella covered other allied communist and non communist states outside of the Warsaw Pact at one time or another besides China prior to the Sino-Soviet Split (see China and weapons of mass destruction) and Cuba (see the Cuban Missile Crisis).
Russian nuclear umbrella
The term is far less used for Russian nuclear guarantees, but is seen occasionally.
-  Unspecified countries
- Ukraine, requested
-  Kazakhstan
-  Commonwealth of Independent States
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 (July 21, 1999). "Japan Under the US Nuclear Umbrella". Nautilus Institute. Archived from the original on April 22, 2008. Retrieved December 4, 2007.
- "The US Nuclear Umbrella Over South Korea". the Nuclear Information Project. October 23, 2006. Retrieved December 4, 2007.
- Nicola Butler; Otfried Nassauer & Daniel Plesch (February 1997). "Extending the Nuclear Umbrella:Undermining the Nuclear Non-Proliferation Treaty". Berlin Information-center for Transatlantic Security. Retrieved December 4, 2007.
- Hyland, Tom (July 5, 2008). "When Australia had a bombshell for US". The Age. Retrieved December 3, 2018.
- Johnson, Thom Shanker and Ian. "U.S.-Japan Missile Defense Accord Is Criticized in China". Retrieved December 3, 2018.
- Engdahl, F. William. "Putin's Gas: Ukraine Gas Dispute - Has Putin Gone Nuts?". www.engdahl.oilgeopolitics.net. Oil Geopolitics. Retrieved December 3, 2018.
- Baker Spring (October 3, 2004). "Finally, U.S. Gets a Nuclear Umbrella". Heritage Foundation. Retrieved December 4, 2007.