No first use
No first use (NFU) refers to a pledge or a policy by a nuclear power not to use nuclear weapons as a means of warfare unless first attacked by an adversary using nuclear weapons. Earlier, the concept had also been applied to chemical and biological warfare.
NATO has repeatedly rejected calls for adopting NFU policy, arguing that preemptive nuclear strike is a key option. In 1993, Russia dropped a pledge given by the former Soviet Union not to use nuclear weapons first. In 2000, a Russian military doctrine stated that Russia reserves the right to use nuclear weapons "in response to a large-scale conventional aggression".
Countries pledging no-first-use
China is the first to propose and pledge NFU policy when it first gained nuclear capabilities in 1964, stating "not to be the first to use nuclear weapons at any time or under any circumstances". Nonetheless, some scholars and observers have questioned the credibility of China's NFU policy. For instance, China had reportedly considered nuclear strikes against the Soviet Union in the event of a conventional Soviet attack. However, China has repeatedly re-affirmed its no-first-use policy in recent years, doing so in 2005, 2008, 2009 and again in 2011. In 2010, the Pentagon concluded that although there is "some ambiguity over the conditions under which China's [no-first-use] policy would or would not apply...there has been no indication that national leaders are willing to attach such nuances and caveats to China's 'no first use' doctrine"
India adopted a "no first use policy" after its nuclear tests in 1998. India's nuclear policy currently states that even though there will be no first-use of nuclear weapons by India, "nuclear retaliation to a first strike will be massive and designed to inflict unacceptable damage".
India's NSA Shivshankar Menon signaled a significant shift from "no first use" to "no first use against non-nuclear weapon states" in a speech on the occasion of Golden Jubilee celebrations of the National Defence College in New Delhi on October 21, 2010, a doctrine Menon said reflected India's "strategic culture, with its emphasis on minimal deterrence". In April 2013 Shyam Saran, convener of the National Security Advisory Board, affirmed that regardless of the size of a nuclear attack against India, be it a miniaturized version or a "big" missile, India will retaliate massively. This shift was earlier implied as Pakistani development of tactical battlefield nuclear weapon, would nullify Indian "no first use" doctrine. 
Countries pledging only to use nuclear weapons defensively
Pakistan, Russia, the United Kingdom, the United States, and France say they will use nuclear weapons against either nuclear or non-nuclear states only in the case of invasion or other attack against their territory or against one of their allies. Historically, NATO military strategy, taking into account the numerical superiority of Warsaw Pact conventional forces, assumed that the use of tactical nuclear weapons would have been required in defeating a Soviet invasion.
In March 2002, British defence secretary Geoff Hoon stated that the UK was prepared to use nuclear weapons against "rogue states" such as Iraq if they ever used "weapons of mass destruction" against British troops in the field. This policy was restated in February 2003.
The U.S. doctrine for the use of nuclear weapons was revised most recently in the Nuclear Posture Review, released April 6, 2010. The 2010 Nuclear Posture review reduces the role of U.S. nuclear weapons, stating that
"The fundamental role of U.S. nuclear weapons, which will continue as long as nuclear weapons exist, is to deter nuclear attack on the United States, our allies, and partners."
The U.S. doctrine also includes the following assurance to other states:
"The United States will not use or threaten to use nuclear weapons against non-nuclear weapons states that are party to the NPT and in compliance with their nuclear non-proliferation obligations."
For states eligible for this assurance, the United States would not use nuclear weapons in response to a chemical or biological attack, but states that those responsible for such an attack would be held accountable and would face the prospect of a devastating conventional military response. Even for states not eligible for this assurance, the United States would only consider the use of nuclear weapons in extreme circumstances to defend the vital interests of the United States or its allies and partners. The Nuclear Posture Review also notes that:
"It is in the U.S. interest and that of all other nations that the nearly 65-year record of nuclear non-use be extended forever."
This supersedes the doctrine of the Bush Administration set forth in "Doctrine for Joint Nuclear Operations" and written under the direction of Air Force General Richard B. Myers, chairman of the Joint Chiefs of Staff. The new doctrine envisions commanders requesting presidential approval to use nuclear weapons to preempt an attack by a nation or a terrorist group using weapons of mass destruction. The draft also includes the option of using nuclear weapons to destroy known enemy stockpiles of nuclear, biological, or chemical weapons.
Although Israel does not officially confirm or deny having nuclear weapons, the country is widely believed to be in possession of them. Its continued ambiguity stance puts it in a difficult position, since to issue a statement pledging 'no first use' would confirm their possession of nuclear weapons.
Israel has said that it "would not be the first country in the Middle East to formally introduce nuclear weapons into the region."
If Israel's very existence is threatened, some speculate that Israel would use a "Samson Option," a "last resort" deterrence strategy of massive retaliation with nuclear weapons, may be initiated should the state of Israel be substantially damaged and/or near destruction. 
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