The United States developed the first atomic weapons during World War II in co-operation with the United Kingdom and Canada as part of the Manhattan Project, out of the fear that Nazi Germany would develop them first. It tested the first nuclear weapon in 1945 ("Trinity"), and remains the only country to have used nuclear weapons against another nation, during the atomic bombings of Hiroshima and Nagasaki. It was the first nation to develop the hydrogen bomb, testing an experimental version in 1952 ("Ivy Mike") and a deployable weapon in 1954 ("Castle Bravo"). Throughout the Cold War it continued to modernize and enlarge its nuclear arsenal, but from 1992 on has been involved primarily in a program of Stockpile stewardship.

  The Soviet Union tested its first nuclear weapon ("Joe-1") in 1949, in a crash project developed partially with espionage obtained during and after World War II (see: Soviet atomic bomb project). The USSR was the second nation to have developed and tested a nuclear weapon. The direct motivation for their weapons development was the development of a balance of power during the Cold War. It tested its first megaton-range hydrogen bomb in 1955 ("RDS-37"). The Soviet Union also tested the most powerful explosive ever detonated by humans, ("Tsar Bomba"), with a theoretical yield of 100 megatons, intentionally reduced to 50 when detonated. After its dissolution in 1991, the Soviets' weapons entered officially into the possession of Russia.

  The United Kingdom tested its first nuclear weapon ("Hurricane") in 1952, drawing largely on data gained while collaborating with the United States during the Manhattan Project. The United Kingdom was the third country in the world after the USA and USSR to develop and test a nuclear weapon. Its programme was motivated to have an independent deterrent against the USSR, while also maintaining its status as a great power. It tested its first hydrogen bomb in 1957, making it the third country to do so after the USA and USSR. The UK maintained a fleet of V-bomber strategic bombers and ballistic missile submarines equipped with nuclear weapons during the Cold War. It currently maintains a fleet of four 'Vanguard' class ballistic missile submarines equipped with Trident IISLBMs. The British government announced a replacement to the current system to take place between 2007-2024. 

  France tested its first nuclear weapon in 1960 ("Gerboise Bleue"), based mostly on its own research. It was motivated by the Suez Crisis diplomatic tension vis-à-vis both the USSR and the Free World allies United States and United Kingdom. It was also relevant to retain great power status, alongside the United Kingdom, during the post-colonial Cold War. France tested its first hydrogen bomb in 1968 ("Opération Canopus"). After the Cold War, France has disarmed 175 warheads with the reduction and modernization of its arsenal that has now evolved to a dual system based on submarine-launched ballistic missiles (SSBN) and medium-range air-to-surface missiles (Rafale fighter-bombers). However new nuclear weapons are in development and reformed nuclear squadrons were trained during Enduring Freedom operation in Afghanistan. In January 2006, President Jacques Chirac stated a terrorist act or the use of weapons of mass destruction against France would result in a nuclear counterattack.

  China tested its first nuclear weapon device in 1964 ("596") at the Lop Nur test site. The weapon was developed as a deterrent against both the United States and the Soviet Union. China would manage to develop a fission bomb capable of being put onto a nuclear missile only two years after its first detonation. It tested its first hydrogen bomb in 1967 ("Test No. 6"), a mere 32 months after testing its first nuclear weapon (the shortest fission-to-fusion development known in history). The country is currently thought to have had a stockpile of around 240 warheads, though because of the limited information available, estimates range from 100 to 400. China is the only nuclear weapons state to give an unqualified negative security assurance to non-nuclear weapon states and the only one to adopt a "no first use" policy.

  India is not a member of the Nuclear Non-Proliferation Treaty. India tested what it called a "peaceful nuclear explosive" in 1974 (which became known as "Smiling Buddha"). The test was the first test developed after the creation of the NPT, and created new questions about how civilian nuclear technology could be diverted secretly to weapons purposes (dual-use technology). India's secret development caused great concern and anger particularly from nations that had supplied it nuclear reactors for peaceful and power generating needs such as Canada. It appears to have been primarily motivated as a general deterrent, as well as an attempt to project India as regional power. India later tested weaponized nuclear warheads in 1998 ("Operation Shakti"), including a thermonuclear device.

   In July 2005, U.S. President George W. Bush and Indian Prime Minister Manmohan Singh announced plans to conclude a Indo-US civilian nuclear agreement. This came to fruition through a series of steps that included India’s announced plan to separate its civil and military nuclear programs in March 2006, the passage of the United States-India Peaceful Atomic Energy Cooperation Act by the U.S. Congress in December 2006, the conclusion of a U.S.-India nuclear cooperation agreement in July 2007, approval by the IAEA of an India-specific safeguards agreement, agreement by the Nuclear Suppliers Group to a waiver of export restrictions for India, approval by the U.S. Congress and culminating in the signature of U.S.-India agreement for civil nuclear cooperation in October 2008. The U.S. State Department said it made it "very clear that we will not recognize India as a nuclear-weapon state“. The United States is bound by the Hyde Act with India and may cease all cooperation with India if India detonates a nuclear explosive device. The US had further said it is not its intention to assist India in the design, construction or operation of sensitive nuclear technologies through the transfer of dual-use items. In establishing an exemption for India, the Nuclear Suppliers Group reserved the right to consult on any future issues which might trouble it. As of September 2009, India was estimated to have had a stockpile of around 60-80 warheads. In addition, Defense News reported in their November 1, 2004 edition, that "[an Indian] Defence Ministry source told Defense News in late 2004 that in the next five to seven years India will have 300–400 nuclear and thermonuclear weapons distributed to air, sea, and land forces." It has estimated that India currently possesses enough separated plutonium to produce and maintain an arsenal of 1,000-2,000 warheads. 

  Pakistan is not a member of the Nuclear Non-Proliferation Treaty either. Pakistan covertly developed nuclear weapons over many decades, beginning in the late 1970s. Pakistan first delved into nuclear power after the establishment of its first nuclear power plant near Karachi with equipment and materials supplied mainly by western nations in the early 1970s. Pakistani Prime Minister Zulfiqar Ali Bhutto promised in 1965 that if India built nuclear weapons Pakistan would too, "even if we have to eat grass." The United States continued to certify that Pakistan did not possess nuclear weapons until 1990, when sanctions were imposed under the Pressler Amendment, requiring a cutoff of U.S. economic and military assistance to Pakistan. In 1998, Pakistan conducted its first six nuclear tests at the Chagai Hills, in response to the five tests conducted by India a few weeks before. Over the years, Pakistan has developed into a crucial nuclear power. It's also alleged that Pakistan is still drastically increasing its nuclear stockpile.

  North Korea was a member of the Nuclear Non-Proliferation Treaty, but announced a withdrawal on January 10, 2003 after the United States accused it of having a secret uranium enrichment program and cut off energy assistance under the 1994 Agreed Framework. In February 2005 they claimed to possess functional nuclear weapons, though their lack of a test at the time led many experts to doubt the claim. However, in October 2006, North Korea stated that due to growing intimidation by the USA, it would conduct a nuclear test to confirm its nuclear status. North Korea reported a successful nuclear test on October 9, 2006 (see 2006 North Korean nuclear test). Most U.S. intelligence officials believe that North Korea did, in fact, test a nuclear device due to radioactive isotopes detected by U.S. aircraft; however, most agree that the test was probably only partially successful. The yield may have been less than a kiloton, which is much smaller than the first successful tests of other powers; however, boosted fission weapons may have an unboosted yield in this range, which is sufficient to start deuterium-tritium fusion in the boost gas at the center; the fast neutrons from fusion then ensure a full fission yield. North Korea conducted a second, higher yield test on May 25, 2009

  Israel is not a member of the Nuclear Non-Proliferation Treaty and refuses to officially confirm or deny having a nuclear arsenal, or having developed nuclear weapons, or even having a nuclear weapons program. Israel has pledged not to be the first country to introduce nuclear weapons into the region, but is also pursuing a policy of strategic ambiguity with regard to their possession. In the late 1960s, Israeli Ambassador to the US Yitzhak Rabin informed the United States State Department, that its understanding of "introducing" such weapons meant that they would be tested and publicly declared, while merely possessing the weapons did not constitute "introducing" them. Although Israel claims that the Negev Nuclear Research Center near Dimona is a "research reactor", or, as was originally claimed, a "textile factory," no scientific reports based on work done there have ever been published. Extensive information about the program in Dimona was also disclosed by technician Mordechai Vanunu in 1986. According to the Natural Resources Defense Council and the Federation of American Scientists, Israel possesses around 75–200 weapons. Imagery analysts can identify weapon bunkers, mobile missile launchers, and launch sites in satellite photographs. Israel may have tested a nuclear weapon along with South Africa in 1979, but this has never been confirmed

  Under NATO nuclear weapons sharing, the United States has provided nuclear weapons for Belgium, Germany, Italy, the Netherlands, and Turkey to deploy and store. This involves pilots and other staff of the "non-nuclear" NATO states practicing handling and delivering the U.S. nuclear bombs, and adapting non-U.S. warplanes to deliver U.S. nuclear bombs. Until 1984 Canada also received shared nuclear weapons, and until 2001, Greece. Members of the Non-Aligned Movement have called on all countries to "refrain from nuclear sharing for military purposes under any kind of security arrangements.” The Institute of Strategic Studies Islamabad (ISSI) has criticized the arrangement for allegedly violating Article I and II of the NPT, arguing that "these Articles do not permit the NWS to delegate the control of their nuclear weapons directly or indirectly to others.” NATO has argued that the weapons' sharing is compliant with the NPT because "the U.S. nuclear weapons based in Europe are in the sole possession and under constant and complete custody and control of the United States."

Nuclear weapons have been present in many nations, often as staging grounds under control of other powers. However, in only a few instances have nations given up nuclear weapons after being in control of them; in most cases this has been because of special political circumstances. The fall of the USSR, for example, left several former Soviet-bloc countries in possession of nuclear weapons.

  South Africa produced six nuclear weapons in the 1980s, but disassembled them in the early 1990s. In 1979, there was a putative detection of a clandestine nuclear test in the Indian Ocean, and it has long been speculated that it was possibly a test by South Africa, perhaps in collaboration with Israel, though this has never been confirmed (see Vela Incident). South Africa signed the Nuclear Non-Proliferation Treaty in 1991]

Former Soviet countries

 Belarus had 81 single warhead missiles stationed on its territory after the Soviet Union collapsed in 1991. They were all transferred to Russia by 1996. Belarus has signed the Nuclear Non-Proliferation Treaty.

 Kazakhstan inherited 1,400 nuclear weapons from the Soviet Union, and transferred them all to Russia by 1995. Kazakhstan has signed the Nuclear Non-Proliferation Treaty.

 Ukraine has signed the Nuclear Non-Proliferation Treaty. Ukraine inherited about 5,000 nuclear weapons when it became independent from the USSR in 1991, making its nuclear arsenal the third-largest in the world. By 1996, Ukraine had voluntarily disposed of all nuclear weapons within its territory, transferring them to Russia.

Former NATO nuclear weapons sharing countries

 Canada Under NATO nuclear weapons sharing, Canada hosted nuclear weapons until 1984.

 Greece Under NATO nuclear weapons sharing, Greece hosted nuclear weapons until 2001.