Nuclear weapons of the United Kingdom in the context of "Treaty on the Non-Proliferation of Nuclear Weapons"

A thermonuclear weapon, fusion weapon or hydrogen bomb (H-bomb) is a second-generation nuclear weapon, utilizing nuclear fusion. The most destructive weapons ever created, their yields typically exceed first-generation nuclear weapons by twenty times, with far lower mass and volume requirements. Characteristics of fusion reactions can make possible the use of non-fissile depleted uranium as the weapon's main fuel, thus allowing more efficient use of scarce fissile material. Its multi-stage design is distinct from the usage of fusion in simpler boosted fission weapons. The first full-scale thermonuclear test (Ivy Mike) was carried out by the United States in 1952, and the concept has since been employed by at least the five NPT-recognized nuclear-weapon states: the United States, Russia, the United Kingdom, China, and France.

The design of all thermonuclear weapons is believed to be the Teller–Ulam configuration. This relies on radiation implosion, in which X-rays from detonation of the primary stage, a fission bomb, are channelled to compress a separate fusion secondary stage containing thermonuclear fuel, primarily lithium-6 deuteride. During detonation, neutrons convert lithium-6 to helium-4 plus tritium. The heavy isotopes of hydrogen, deuterium and tritium, then undergo a reaction that releases energy and neutrons. For this reason, thermonuclear weapons are often colloquially called hydrogen bombs or H-bombs.

Nuclear proliferation is the spread of nuclear weapons to additional countries, particularly those not recognized as nuclear-weapon states by the Treaty on the Non-Proliferation of Nuclear Weapons, commonly known as the Non-Proliferation Treaty or NPT. Nuclear proliferation occurs through the spread of fissile material, and the technology and capabilities needed to produce it and to design and manufacture nuclear weapons. In a modern context, it also includes the spread of nuclear weapons to non-state actors. Proliferation has been opposed by many nations with and without nuclear weapons, as governments fear that more countries with nuclear weapons will increase the possibility of nuclear warfare (including the so-called countervalue targeting of civilians), de-stabilize international relations, or infringe upon the principle of state sovereignty. Conversely, supporters of deterrence theory argue that controlled proliferation decreases conflict rates via nuclear peace.

Nuclear weapons were initially researched during World War II, jointly by the United States, United Kingdom and Canada, and separately by Germany, Japan, the Soviet Union, and France. The United States was the first and is the only country to have used a nuclear weapon in war, when it used two bombs against Japan in August 1945. After surrendering, Germany and Japan ceased to be involved in any nuclear weapon research. A nuclear arms race followed, with further countries developing and testing nuclear weapons. The US primarily competed with the Soviet Union, which carried out their first test in 1949. Seven other countries developed nuclear weapons during the Cold War. The UK and France, both NATO members, developed fission and fusion weapons throughout the 1950s, and 1960s, respectively. China developed both against the backdrop of the Sino-Soviet split.

An intercontinental ballistic missile (ICBM) is a ballistic missile with a range greater than 5,500 kilometres (3,400 mi), primarily designed for nuclear weapons delivery (delivering one or more thermonuclear warheads). Conventional, chemical, and biological weapons can also be delivered with varying effectiveness but have never been deployed on ICBMs. Most modern designs support multiple independently targetable reentry vehicles (MIRVs), allowing a single missile to carry several warheads, each of which can strike a different target. The United States, Russia, China, France, India, the United Kingdom, Israel, and North Korea are the only countries known to have operational ICBMs. Pakistan is the only nuclear-armed state that does not possess ICBMs.

Early ICBMs had limited precision, which made them suitable for use only against the largest targets, such as cities. They were seen as a "safe" basing option, one that would keep the deterrent force close to home where it would be difficult to attack. Attacks against military targets (especially hardened ones) demanded the use of a more precise, crewed bomber. Second- and third-generation designs (such as the LGM-118 Peacekeeper) dramatically improved accuracy to the point where even the smallest point targets can be successfully attacked.

There are currently nine sovereign states that are generally understood to possess nuclear weapons, though only eight formally acknowledge possessing them. In order of first successful nuclear test, the world's nine nuclear-armed states are the United States (1945), Russia (1949), the United Kingdom (1952), France (1960), China (1964), India (1974), Pakistan (1998), and North Korea (2006); Israel is believed to have acquired nuclear weapons around 1967, but has never openly tested or formally acknowledged having them. Under the Non-Proliferation Treaty (NPT), the United States, Russia, the United Kingdom, France, and China are recognized "nuclear-weapons states" (NWS). They are also the five permanent members of the United Nations Security Council. Israel, India, and Pakistan never signed the NPT, while North Korea acceded to it in 1985 before announcing withdrawal in 2003.

South Africa developed nuclear weapons by 1982 but dismantled them around 1989 and joined the NPT in 1991. Following the dissolution of the Soviet Union in 1991, the former Soviet republics of Ukraine, Kazakhstan, and Belarus had nuclear weapons deployed on their territories, but agreed to transfer them to Russia (which inherited the Soviet Union's international rights and obligations) and join the NPT as non-nuclear-weapon states.

Nuclear sharing is a concept in deterrence theory in which a nuclear-armed country deploys nuclear weapons on the territory of a country that does not possess nuclear weapons and extended its nuclear deterrent to that country. Nuclear sharing typically also involves joint planning and training processes for potentially using them, going beyond nuclear stationing or nuclear basing, which refer to a nuclear-armed country's deployment of nuclear weapons on foreign soil without an operational role for the host country's military and government.

It was originally conceived during the Cold War, when the United States and the Soviet Union placed their own nuclear weapons in many non-nuclear countries of the American-aligned First World and the Soviet-aligned Second World, as part of the nuclear arms race between the two sides. However, since the dissolution of the Soviet Union in 1991, the concept continues to be practiced by the United States and Russia. United States nuclear weapons, for delivery by fighter aircraft, are based in Belgium, Germany, Italy, the Netherlands, Turkey, and the United Kingdom. Russian nuclear weapons, for delivery by aircraft and short-range missiles, are deployed in Belarus.