A nuclear war is an armed conflict in which one or more parties detonate nuclear weapons, releasing energy millions of times more powerful than conventional explosives. The world currently holds an estimated 12,241 nuclear warheads, with about 3,912 of them actively deployed on missiles and at military bases, ready to launch. Even a limited exchange between two countries could trigger global consequences lasting more than a decade, while a full-scale war between major powers would threaten human civilization itself.
Strategic vs. Tactical Nuclear Weapons
Nuclear weapons fall into two broad categories based on their intended use. Strategic nuclear weapons are designed to cross continents, carried by intercontinental ballistic missiles, submarine-launched missiles, and long-range bombers. Their explosive yields range from about 100 kilotons to over a megaton, and their purpose is deterrence through the threat of massive, devastating retaliation. For reference, the bomb dropped on Hiroshima was about 15 kilotons.
Tactical nuclear weapons are smaller, designed for use on or near a battlefield rather than against distant cities. Their yields range from fractions of a kiloton up to about 50 kilotons, and their delivery systems typically travel under 310 miles. These can be deployed by ships, planes, short-range ballistic missiles, torpedoes, and ground forces. Russia’s Iskander-M mobile missile system, for example, can carry either a conventional or nuclear warhead. The distinction matters because tactical weapons lower the threshold for nuclear use: they’re built not for global annihilation, but for gaining an advantage in a specific fight, which makes the decision to use one feel more “manageable” to military planners.
What a Single Detonation Does
A nuclear explosion produces three immediate killing mechanisms: a shockwave, a pulse of extreme heat, and a burst of radiation. For a 10-kiloton weapon detonated at ground level (smaller than many warheads in service today), severe shockwave damage extends about half a mile from the blast site. The thermal pulse, intense enough to cause fatal burns and ignite fires, reaches roughly a mile out. Flying debris can injure or kill people up to a few miles away.
The initial burst of nuclear radiation is lethal to unprotected people within about three-quarters of a mile. Beyond that radius, radioactive fallout carried by wind creates an irregular, elongated zone of contamination stretching up to 6 miles downwind, where radiation doses can still be fatal. A larger weapon, in the hundreds of kilotons, would multiply all of these distances significantly.
Radiation Sickness and Its Stages
People exposed to high doses of radiation develop acute radiation syndrome, which unfolds in stages depending on how much radiation the body absorbs. At lower but still dangerous doses, the bone marrow is the first system to fail. The body stops producing enough blood cells, leading to infections and uncontrolled bleeding. Roughly half of people exposed to moderate doses in this range die within 60 days without medical treatment.
At higher doses, the lining of the digestive tract breaks down, causing severe vomiting, diarrhea, and internal bleeding. Death typically follows within two weeks, and no treatment can reverse it. At extreme doses, the cardiovascular and nervous systems collapse, and death occurs within three days. In a nuclear war, medical systems would be overwhelmed or destroyed in the affected areas, meaning most radiation victims would receive little or no care.
Radioactive Fallout
Fallout contains hundreds of different radioactive particles. Some decay within minutes or days, while others persist for decades. The most significant long-term contaminants include cesium-137, which has a half-life of about 30 years, strontium-90, and iodine-131 (half-life of 8 days, but dangerous in the immediate aftermath because the thyroid gland absorbs it readily).
The health risk depends on the type of radiation and how you’re exposed. Gamma rays travel long distances through air and penetrate the body, making them dangerous even from a distance. They can only be blocked by heavy shielding like thick concrete or lead. Alpha and beta particles don’t travel far and can’t penetrate skin easily, but they become far more dangerous when inhaled or swallowed with contaminated food and water. Once inside the body, these particles damage cells directly and increase cancer risk over time. This is why fallout’s danger extends far beyond the blast zone, contaminating agricultural land, water supplies, and ecosystems across enormous areas.
Nuclear Winter and Global Climate Effects
The most catastrophic consequence of a large-scale nuclear war wouldn’t be the explosions themselves. It would be what happens to the atmosphere afterward. Burning cities would inject massive quantities of soot into the upper atmosphere, where it would block sunlight for years. Climate models examining a war using roughly one-third of current arsenals project that 150 million tons of soot would cause surface temperatures to plummet far below freezing across much of the globe. Global precipitation would drop by 45%, devastating agriculture worldwide.
Even a scenario producing 50 million tons of soot (a smaller but still major exchange) would cause rapid drops in temperature, rainfall, and sunlight reaching the surface, threatening global food production for at least a year. This is the scenario scientists call “nuclear winter,” and modern climate models confirm the conclusions first proposed in the 1980s: a large nuclear war would starve billions of people who survived the initial blasts, including those in countries far from the conflict.
Ozone Layer Destruction
Nuclear detonations and the resulting fires would also shred the ozone layer, which shields Earth from ultraviolet radiation. A full-scale nuclear war would destroy up to 75% of the global ozone layer, with the damage persisting for roughly 15 years. Even a regional nuclear war (such as a conflict between India and Pakistan) would reduce ozone by about 25%, taking 12 years to recover.
Initially, the soot blocking sunlight would also block UV radiation at ground level. But as the soot gradually clears and the ozone remains depleted, UV exposure would spike to extreme levels. In the tropics, UV Index values would exceed 35 for four years. For context, a UV Index above 11 is considered “extreme” on a normal day. In the southern polar regions, summer UV values would top 45 for three years. This level of UV radiation would damage crops, kill marine plankton (a foundation of ocean food chains), and cause widespread skin and eye damage in humans and animals.
Who Has Nuclear Weapons
Nine countries possess nuclear weapons as of January 2025. The United States and Russia dominate the count, holding roughly 90% of the world’s arsenal between them. The U.S. maintains a military stockpile of about 5,328 warheads, with 1,770 actively deployed. Russia holds approximately 5,580, with 1,718 deployed.
The remaining nuclear states hold far smaller arsenals. China has an estimated 500 warheads in its military stockpile, with only 24 currently deployed on missiles or at bases. France maintains 290, the United Kingdom 225, India 180, Pakistan 170, Israel 90, and North Korea an estimated 50. India, Pakistan, North Korea, and Israel do not have any warheads classified as “deployed” in the same sense as the larger powers, though they could prepare them for use.
Arms Control and Its Current Status
The primary framework limiting the two largest arsenals has been the New START Treaty between the United States and Russia. It entered into force in 2011 and placed verifiable caps on deployed intercontinental-range nuclear weapons. Both countries met the treaty’s central limits by 2018 and have remained at or below them since. The treaty was extended in 2021 for an additional five years, keeping it in force through February 4, 2026.
With that expiration approaching and no successor agreement in place, the world faces the real possibility of having no binding limits on U.S. and Russian nuclear arsenals for the first time in decades. Russia suspended its participation in the treaty in 2023, further undermining the verification mechanisms that allowed each side to confirm the other’s compliance. Meanwhile, China is rapidly expanding its stockpile from a historically small base, and no existing treaty covers its weapons. The combination of expiring agreements, suspended cooperation, and expanding arsenals marks one of the most unstable periods in nuclear arms control since the Cold War.