At the dawn of the nuclear age, the United States hoped to maintain a monopoly on its new weapon, but the secrets and the technology for building the atomic bomb soon spread. The United States conducted its first nuclear test explosion in July 1945 and dropped two atomic bombs on the cities of Hiroshima and Nagasaki, Japan, in August 1945. Just four years later, the Soviet Union conducted its first nuclear test explosion. The United Kingdom (1952), France (1960), and China (1964) followed. Seeking to prevent the nuclear weapon ranks from expanding further, the United States and other like-minded countries negotiated the nuclear Nonproliferation Treaty (NPT) in 1968 and the Comprehensive Nuclear Test Ban Treaty (CTBT) in 1996.
India, Israel, and Pakistan never signed the NPT and possess nuclear arsenals. Iraq initiated a secret nuclear program under Saddam Hussein before the 1991 Persian Gulf War. North Korea announced its withdrawal from the NPT in January 2003 and has successfully tested advanced nuclear devices since that time. Iran and Libya have pursued secret nuclear activities in violation of the treaty’s terms, and Syria is suspected of having done the same. Still, nuclear nonproliferation successes outnumber failures, and dire decades-old forecasts that the world would soon be home to dozens of nuclear-armed have not come to pass.
At the time the NPT was concluded, the nuclear stockpiles of both the United States and the Soviet Union/Russia numbered in the tens of thousands. Beginning in the 1970s, U.S. and Soviet/Russian leaders negotiated a series of bilateral arms control agreements and initiatives that limited, and later helped to reduce, the size of their nuclear arsenals.
Today, the United States deploys 1,357 and Russia deploys 1,456 strategic warheads on several hundred bombers and missiles, and are modernizing their nuclear delivery systems. Warheads are counted using the provisions of the New START agreement, which was extended for 5 years in January 2021.
New START caps each country at 1,550 strategic deployed warheads and attributes one deployed warhead per deployed heavy bomber, no matter how many warheads each bomber carries. Warheads on deployed ICBMs and SLBMs are counted by the number of re-entry vehicles on the missile. Each re-entry vehicle can carry one warhead.
Both Russia and China also possess smaller numbers of non-strategic (a.k.a. tactical) nuclear warheads, which are not subject to any treaty limits.
China, India, and Pakistan are all pursuing new ballistic missile, cruise missile, and sea-based nuclear delivery systems. In addition, Pakistan has lowered the threshold for nuclear weapons use by developing tactical nuclear weapons capabilities to counter perceived Indian conventional military threats. North Korea continues its nuclear pursuits in violation of its earlier denuclearization pledges.
The world's nuclear-armed states possess a combined total of about 13,080 nuclear warheads. North Korea, the ninth nuclear-weapon state, is estimated to have produced enough fissile material for 40-50 warheads, although the actual size of its stockpile remains unknown.