Nuclear warfare is armed conflict that involves the detonation of nuclear weapons, devices that release energy through splitting atoms (fission) or fusing them together (fusion). A single modern warhead can destroy an entire city, and the nine countries that possess these weapons hold a combined military stockpile of roughly 9,600 warheads as of January 2025. The concept shapes global politics, military strategy, and international law in ways no other weapon class does.
How Nuclear Weapons Work
Nuclear weapons produce their destructive force through nuclear reactions rather than chemical explosions. A fission weapon splits heavy atoms like uranium or plutonium, releasing enormous energy from a relatively small amount of material. A fusion weapon, sometimes called a thermonuclear or hydrogen bomb, uses a fission explosion to trigger the fusion of light atoms, producing a far larger blast. The bomb dropped on Hiroshima in 1945 had a yield of about 15 kilotons, meaning it released energy equivalent to 15,000 tons of conventional explosive. That single weapon killed an estimated 70,000 to 80,000 people instantly.
Modern warheads dwarf that figure. Some carry yields in the hundreds of kilotons; others are deliberately smaller but far more accurate. The B61-12, a gravity bomb in the current U.S. arsenal, has a variable yield that can be dialed from as low as 0.3 kilotons up to 50 kilotons, depending on the target. A planned earth-penetrating variant would reach 360 kilotons, roughly 24 times the Hiroshima bomb.
Tactical vs. Strategic Weapons
Nuclear weapons fall into two broad categories based on how they would be used. Strategic nuclear weapons are the high-yield warheads designed to destroy cities, industrial centers, or an enemy’s own nuclear forces. These are the weapons covered by arms control treaties and are typically delivered by intercontinental ballistic missiles, submarine-launched missiles, or long-range bombers.
Tactical nuclear weapons, sometimes called non-strategic weapons, are lower-yield devices intended for use on a battlefield. They include nuclear artillery shells, torpedoes, mines, and short-range gravity bombs. There is no universally agreed definition separating the two categories, and the distinction can blur: a “low-yield” weapon with the explosive power of the Hiroshima bomb would still devastate any city it hit. The classification matters mainly for treaty negotiations and military planning.
The Nuclear Triad
Major nuclear powers structure their arsenals around three delivery systems, collectively called the nuclear triad. Each leg of the triad serves a different strategic purpose, and together they make it extremely difficult for an adversary to destroy all of a country’s nuclear weapons in a first strike.
- Land-based missiles: Intercontinental ballistic missiles (ICBMs) sit in hardened underground silos and can reach targets within 30 minutes of launch. The U.S. maintains 400 single-warhead ICBMs across sites in the western United States.
- Submarine-launched missiles: Ballistic missile submarines patrol the oceans and are nearly impossible to track. The U.S. operates 14 submarines carrying a total of 240 missiles. Because submarines can hide beneath the ocean surface, they serve as the most survivable part of the triad.
- Bombers: Nuclear-capable aircraft like the B-52H and the B-2 stealth bomber can carry gravity bombs and cruise missiles with nuclear warheads. Bombers are the slowest delivery method but offer the most flexibility, since they can be recalled after launch.
What a Nuclear Detonation Does
A nuclear explosion produces four overlapping waves of destruction. The initial flash releases intense thermal radiation, temperatures hot enough to ignite fires and cause severe burns miles from the blast center. A massive shockwave follows within seconds, leveling buildings and generating winds far stronger than any hurricane. Prompt ionizing radiation, a burst of gamma rays and neutrons, hits within the first minute. For large weapons, the blast and heat zones extend well beyond the radiation zone, meaning most casualties come from the shockwave and fires rather than direct radiation exposure.
Then comes fallout. When a weapon detonates at or near ground level, it draws millions of tons of soil and debris into the fireball, irradiating the material and lofting it into the atmosphere. Larger radioactive particles settle back to earth within the first 24 hours, blanketing a downwind area that can stretch dozens or hundreds of miles depending on wind patterns and weapon yield. This local fallout is the most immediately dangerous form, delivering intense radiation doses to anyone without shelter. Finer particles rise into the upper atmosphere and circulate globally, depositing lower levels of radiation over weeks to months. Weapons detonated high in the air produce very little local fallout because no ground debris is pulled into the fireball, but they still contribute to this longer-term global contamination.
Nuclear Winter
Beyond the immediate destruction, a large-scale nuclear exchange would reshape the global climate. The landmark 1983 study published in Science by Carl Sagan and colleagues modeled what would happen if thousands of megatons of nuclear weapons detonated over cities. Massive firestorms would inject soot and dust into the upper atmosphere, where it would spread around the planet within one to two weeks. Average light levels at the surface could drop to just a few percent of normal, and land temperatures could plunge to between negative 15 and negative 25 degrees Celsius.
Even a more limited exchange carries serious risk. The models showed that as few as 100 megatons detonated over major urban centers could produce enough smoke to block significant sunlight for weeks and cause subfreezing temperatures over land for months, even during summer. The resulting disruption to agriculture and ecosystems would threaten food supplies for billions of people far from any blast zone.
Who Has Nuclear Weapons
Nine countries are known or believed to possess nuclear weapons. According to the Stockholm International Peace Research Institute, their stockpiles as of January 2025 break down as follows:
- Russia: 4,380 warheads in military stockpile, 1,718 deployed
- United States: 3,708 in stockpile, 1,770 deployed
- China: 600 in stockpile, 24 deployed
- France: 290 in stockpile, 280 deployed
- United Kingdom: 225 in stockpile, 120 deployed
- India: 180 in stockpile
- Pakistan: 170 in stockpile
- Israel: 90 in stockpile (Israel neither confirms nor denies its arsenal)
- North Korea: approximately 50 in stockpile
The global total is roughly 12,241 warheads including retired weapons awaiting dismantlement. Russia and the United States together hold about 88 percent of the world’s nuclear weapons. China’s stockpile has been growing significantly in recent years, though it remains far smaller than either superpower’s arsenal.
Deterrence and Mutual Assured Destruction
The central logic behind nuclear arsenals is deterrence: the idea that no rational leader would start a nuclear war if doing so guaranteed their own country’s destruction. This concept, known as mutual assured destruction (MAD), emerged during the Cold War as both the United States and Soviet Union built enough weapons to annihilate each other several times over. The theory requires each side to maintain a “second-strike capability,” meaning enough weapons must survive an enemy’s initial attack to deliver a devastating response. Submarine-based missiles are the backbone of this guarantee, since they are virtually undetectable at sea.
MAD has held since 1945. No nuclear weapon has been used in conflict since the bombings of Hiroshima and Nagasaki. Critics of deterrence theory point out that it depends on rational decision-making, accurate intelligence, and functioning communication systems, all of which could fail during a crisis. Accidental launches, miscommunication, or the actions of a leader who does not fear retaliation all represent scenarios where deterrence could break down.
Arms Control and Treaties
Several treaties have attempted to limit nuclear arsenals and prevent their spread. The Treaty on the Non-Proliferation of Nuclear Weapons, in force since 1970, commits non-nuclear states to forgo developing weapons in exchange for access to peaceful nuclear technology, while nuclear-armed states pledge to work toward disarmament. The Treaty on the Prohibition of Nuclear Weapons, adopted in 2017, bans nuclear weapons outright, but none of the nine nuclear-armed states have signed it.
The most significant bilateral agreement has been New START, which entered into force in 2011 between the United States and Russia. It caps each side’s deployed strategic warheads and establishes verification and inspection protocols. The treaty was extended through February 4, 2026, but Russia suspended its participation in 2023, and no successor agreement is currently in place. The expiration of New START would leave the two largest nuclear powers without any binding limits on their arsenals for the first time since the 1970s.
Why It Matters Now
The global nuclear landscape is shifting in ways that increase risk. China is rapidly expanding its arsenal. Arms control frameworks between the U.S. and Russia are collapsing. Several nuclear-armed states are developing new delivery systems and smaller, more “usable” warheads, which some analysts worry could lower the threshold for actual use. Pakistan and India, both nuclear-armed, share a contested border and have fought multiple conventional wars. North Korea continues testing missiles capable of reaching distant targets.
The core danger of nuclear warfare has not changed since 1945: these are weapons whose use, even on a limited scale, would cause destruction and human suffering on a level that conventional weapons cannot approach, with environmental consequences that would cross every border on earth.