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 over two dozen major allies. They include member states of NATO as well Asia-Pacific countries that have long-standing mutual defence treaties with the US, among them Australia, Japan, South Korea.
Defence white papers issued by Australian governments at least as far back as 1994 have stated that Australia will continue to rely on the US to deter any nuclear threat or attack on Australia.
The latest white paper, published in May, said that stable nuclear deterrence was expected to be a feature of the international system for the foreseeable future, and that in this context extended deterrence would continue to be viable.
It added, ''The challenge will be to deter rogue states of concern, some of which may develop a level of capability in terms of long-range ballistic missiles, coupled potentially with WMD (weapons of mass destruction) warheads.
''Iran and North Korea, and possibly others in the future, will continue to pursue long-range ballistic missile programs that pose a direct, though remote, risk to our own security.''
Extended deterrence was designed not just to protect non-nuclear allies such as Australia 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 program, and its parallel testing of a wide range of missiles that may one day be armed with nuclear warheads as well as the North Korea'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 WMD. In such a situation, how effective is 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. Also in the background is China's rising power and influence, and its often-stated objection to US offers of extended deterrence to allies.
China says that its objection is based on the principle that nuclear weapons should be used solely in self-defence. China believes that the US nuclear umbrella covers Taiwan (which it regards as a renegade province) as well as Japan, its rival for eminence in Asia.
In this climate of uncertainty, some conservative Japanese and South Korean politicians have argued that their countries should have nuclear weapons for self-protection.
If this were to happen, other Asian countries might follow, triggering a nuclear arms race that would destabilise the region and undermine economic growth.
Since 1968, Japan has been formally committed to three non-nuclear principles of not possessing or producing nuclear arms, and not permitting their entry into the country.
In March, Yukio Satoh, a leading Japanese strategic thinker, said that Japan's adherence to these principles depended largely on the credibility of the US-Japan Security Treaty and America's commitment to defend Japan from any offensive action, including nuclear threats.
He added, ''A unique feature of the Japan-US security arrangements is that there have been no consultations on how American extended deterrence should function, nor even any mechanism put in place for such consultations.''
To arrest this dangerous drift, the US sent senior officials to Tokyo for talks last month. They gave an assurance that the US commitment to protect Japan was ''absolutely unshakeable''.
The two sides also agreed to establish an official framework for discussions on how the nuclear umbrella should function and other deterrence measures.
A similar consultation channel is expected to be set up by the US and South Korea.
The US aim is to discourage growth of pro-nuclear sentiment in Asia and send a clear message not only to North Korea but to other potential nuclear proliferators, such as Iran, that any aggressive moves against its neighbours would bring a strong response, including possible use of nuclear weapons.
Whether Japan or South Korea would approve a nuclear response is questionable. Recently South Korea ruled out the redeployment of US nuclear arms on its soil, despite North Korea's nuclear program.
South Korea and the US say that all US nuclear weapons were withdrawn from South Korea in 1991, one year before the two Koreas agreed to keep the peninsula nuclear-free.
However, other measures are being put in place to strengthen Japanese and South Korean defences against possible attack.
Both countries are being provided by the US with extra interceptor rockets that can be fired from land and warships to destroy incoming missiles.
The US is also reminding allies as well as adversaries that although it seeks universal nuclear disarmament, it will keep its weapons for as long as others have them.
The writer is a visiting senior research fellow at the Institute of Southeast Asian Studies in Singapore.
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