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 order to have a credible deterrent that could compensate for the overwhelming conventional weapon superiority enjoyed by the Soviet Army in the Eurasian land mass. 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". This is because the balance of forces was reversed — NATO is now enjoying a clear superiority in conventional weapons.
Countries pledging no-first-use
China became the first nation 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". During the Cold War, China decided to keep the size of its nuclear arsenal small rather than compete in an international arms race with the United States and the Soviet Union. China has repeatedly re-affirmed its no-first-use policy in recent years, doing so in 2005, 2008, 2009 and again in 2011. China has also consistently called on the United States to adopt a no-first-use policy, to reach a NFU agreement bilaterally with China, and to conclude an NFU agreement among the five nuclear weapon states. The United States has repeatedly refused these calls.
India first adopted a "No first use" policy after its second nuclear tests, Pokhran-II, in 1998. In August 1999, the Indian government released a draft of the doctrine which asserts that nuclear weapons are solely for deterrence and that India will pursue a policy of "retaliation only". The document also maintains that India "will not be the first to initiate a nuclear first strike, but will respond with punitive retaliation should deterrence fail" and that decisions to authorise the use of nuclear weapons would be made by the Prime Minister or his 'designated successor(s)'. According to the National Research Development Corporation, despite the escalation of tensions between India and Pakistan in 2001–2002, India remained committed to its nuclear no-first-use policy. India is in the process of developing a nuclear doctrine based on "credible minimum deterrence".
In a speech at the National Defence College on October 21, 2010 by India's then National Security Advisor, Shivshankar Menon, the wording was changed from "no first use" to "no first use against non-nuclear weapon states", although some argued that this was not a substantive change but "an innocent typographical or lexical error in the text of the speech." India’s current PM Modi has in the run up to the recent general elections reiterated commitment to no first use policy. 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 tactical nuclear weapon or a strategic nuclear weapon, India will retaliate massively. This was in response to reports that Pakistan had developed a tactical battlefield nuclear weapon, in an attempt to nullify an Indian "no first use" retaliatory doctrine.
During the 7th Congress of the Workers' Party of Korea in 2016, supreme leader Kim Jong-un stated that North Korea would "not use nuclear weapons first unless aggressive hostile forces use nuclear weapons to invade on our sovereignty".  However, just two months prior, North Korea threatened a preemptive attack against the United States using nuclear weapons.
Countries pledging to use nuclear weapons only 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.
Russia describes its entire military doctrine as defensive (see Military doctrine of 2010). With regard to nuclear weapons specifically, Russia reserves the right to use nuclear weapons
- in response to the use of nuclear and other types of weapons of mass destruction against it or its allies, and also
- in case of aggression against Russia with the use of conventional weapons when the very existence of the state is threatened. 
The new military doctrine of 2014 does not depart from this stance. 
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 United States has refused to adopt a no-first-use policy, saying that it "reserves the right to use" nuclear weapons first in the case of conflict. 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 consider the use of nuclear weapons only in extreme circumstances to defend the vital interests of the United States or its allies and partners. The Nuclear Posture Review also notes:
"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.
Pakistan has a no first attack policy in place since 1971. This policy was reiterated after the nuclear tests in 1998. Pakistan has vowed never to invade or attack another country under any circumstances. Pakistan's foreign minister Shamshad Ahmad had warned that if Pakistan is ever invaded or attacked, it will use "any weapon in its arsenal" to defend itself.
Pakistan refuses to adopt a "no-first-use" doctrine, indicating that it would launch nuclear weapons even if the other side did not use such weapons first. Pakistan's asymmetric nuclear posture has significant influence on India's decision ability to retaliate, as shown in 2001 and 2008 crises, when non-state actors carried out deadly terrorist attacks on India, only to be met with a relatively subdued response from India. A military spokesperson stated that "Pakistan's threat of nuclear first-use deterred India from seriously considering conventional military strikes."
Pakistan's National Security Advisor Sartaj Aziz defended the policy of first use. Aziz stated that Pakistan's first use doctrine is entirely deterrent in nature. He explained that it was effective after the 2001 Indian Parliament attack and argued that if Pakistan had a no-first use policy, there would have been a major war between the two countries.
Although Israel does not officially confirm or deny having nuclear weapons, the country is widely believed to be in possession of them. Its continued ambiguous 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, should the State of Israel be substantially damaged and/or near destruction.
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