In the cold war, the United States protected its allies from possible attack by a nuclear-armed Soviet Union by threatening a devastating nuclear response. This policy became the foundation for extended deterrence.
Although the cold war is long gone, the assurance offered to non-nuclear allies by the so-called US nuclear 'umbrella' remains. It still covers more than two dozen major allies. They include member states of Nato (the North Atlantic Treaty Organisation), as well Asia-Pacific countries that have long-standing mutual defence treaties with the US, among them Japan, South Korea and Australia.
Extended deterrence was designed not just to protect these allies but also to assure them that it was unnecessary to develop their own nuclear weapons. This has helped to limit the number of states known to have nuclear arms to nine - the five nuclear powers acknowledged in the non-proliferation treaty (Britain, China, France, Russia and the US), plus India, Israel, Pakistan and North Korea.
However in Asia, the value of extended deterrence is being called into question by several recent developments. Chief among them is North Korea's detonation of two nuclear explosive devices since 2006, most recently in May, its declared intent to make more nuclear weapons and never abandon the programme, and its parallel testing of a wide range of missiles that may one day be armed with nuclear warheads as well as the North's existing extensive stocks of chemical and biological weapons.
Unlike the former Soviet Union, North Korea is seen by its neighbours as unpredictable and possibly even prepared to use weapons of mass destruction. In such a situation, how effective is a US extended deterrence likely to be and what does it mean in practice?
Would any US response to a North Korean attack involve nuclear arms or only conventional weapons? And could such a response achieve its aim of destroying a leadership and military assets in reinforced shelters deep underground?
North Korean belligerence has been accompanied by another unsettling development: the Obama administration's strong push for nuclear disarmament. While Japan and South Korea welcome eventual abolition of nuclear arms in principle, they worry that it may dilute US willingness and capacity to deter an attack and respond resolutely if it occurs.