Nuclear weapons pose an enormous threat to humanity. Since their very first use, different leaders and organizations have been trying to prevent proliferation to additional countries. Despite their efforts, more states than ever before have obtained nuclear weapons. This timeline explores some of the critical actions and decisions that led to today’s distribution of those weapons and the world’s non-proliferation regime.

The Nuclear Age Begins

Scientific discoveries in the late 1930s made nuclear weapons a possibility for the first time in history. During World War II, the United States and its allies were afraid that their enemies would develop nuclear weapons first, so in 1942 they started the Manhattan Project, a secret research effort led by the U.S. government to develop nuclear weapons. The creation and use of nuclear weapons ushered in the nuclear age, and growing tensions between the United States and the Soviet Union, both armed with nuclear weapons, during the Cold War made the threat of nuclear war a real possibility.

The U.S. military conducts a nuclear test at Bikini Atoll in the Marshall Islands in 1946.

U.S. Navy

The destruction in Hiroshima, Japan after the U.S. dropped a nuclear bomb to end World War II.

Australian War Memorial

First Atomic Bombs Are Dropped on Hiroshima and Nagasaki

On August 6, 1945, the United States dropped the first atomic bomb used in combat on Hiroshima, Japan, killing an estimated 140,000 people. On August 9, the United States dropped the second atomic bomb on Nagasaki, which killed more than seventy thousand people. The death toll from the blasts increased in later years as survivors faced elevated rates of cancer linked to radiation exposure. The bombs wiped out both cities, as ground temperatures swelled to more than 4,000°C (7,000°F) and shockwaves leveled entire communities. Six days after the Nagasaki bombing, Emperor Hirohito of Japan announced his country’s surrender. Those atomic bombs remain the only ones ever used in war.

U.S. President Dwight D. Eisenhower announces the "Atoms for Peace" program in a speech to the United Nations General Assembly on December 8, 1953.

Educational Video Group via YouTube

IAEA Is Created

The International Atomic Energy Agency (IAEA) was created with the mission of promoting and overseeing the peaceful use of nuclear technology. U.S. President Dwight D. Eisenhower’s December 1953 “Atoms for Peace” speech is considered to have created the impetus for forming the institution. Eisenhower said that an international agency was needed to prevent the spread, or proliferation, of nuclear technology, warning that, if unchecked, it could result in “the annihilation of the irreplaceable heritage of mankind.”

A nuclear radiation sign warns of a contaminated area along the Techa River in Russia.

Alla Slapovskaya and Alisa Nikulina/Ecodefense

Kyshtym Nuclear Disaster Occurs In Secret

On September 29, a hastily stored tank of nuclear waste exploded in the Russian town Ozyorsk, the original site of the Soviet nuclear weapons program. The disaster released more radioactive contamination than the 1986 Chernobyl disaster and further contaminated the already heavily polluted area. Unlike other well-known nuclear accidents, this one occurred at a nuclear weapons facility instead of an energy plant, so the Soviet government tried to cover it up. In fact, the disaster was not widely known about until an exiled Soviet scientist reported on it in 1976.

U.S. President John F. Kennedy gives a televised address about the Cuban Missile Crisis from the White House on October 22, 1962.

New York Daily News via YouTube

The Cuban Missile Crisis Threatens Nuclear War

On October 15, a U.S. military plane discovered Soviet nuclear missiles under construction in Cuba, only about one hundred miles from the Florida coast. President John F. Kennedy sent in the U.S. Navy to surround Cuba and demanded that Soviet leader Nikita Khrushchev dismantle the missiles. After several tense days, Khrushchev agreed to remove the missiles in exchange for a public guarantee from the United States that it would not attack Cuba, a Soviet ally. The United States also secretly agreed to remove certain missiles from Turkey, out of range of the Soviet Union. The crisis remains the closest the world has ever come to a nuclear war.

Nuclear Nonproliferation Goes Global

The late 1960s and early 1970s were marked by both progress and setbacks in nuclear nonproliferation worldwide. On one hand, the United Nations established the first framework relating to nuclear weapons with the Nuclear Nonproliferation Treaty (NPT). And the world’s two biggest nuclear powers, the United States and the Soviet Union, took initial steps toward limiting their nuclear arsenals. On the other hand, India obtained nuclear weapons.

Protesters march against nuclear weapons from Caernarfon to Bangor in Wales on March 22, 1962.

Geoff Charles via National Library of Wales

Alfonso García Robles, Mexican diplomat and one of the main drivers of the Treaty of Tlatelolco, speaks about the importance of nuclear disarmament.

Smithsonian Folkways Recordings via YouTube

First Nuclear-Weapon-Free Zone Is Established

The Treaty of Tlatelolco opened for signature on February 14, establishing Latin America as the first nuclear-weapon-free zone (NWFZ): an area in which countries agree to “prohibit and prevent” the “testing, use, manufacture, production, or acquisition by any means whatsoever of any nuclear weapons.” Mexican ambassador and Nobel Peace Prize winner Alfonso Garcia Robles hoped the “territories of powers which possess those horrible tools of mass destruction will become ‘something like contaminated islets subjected to quarantine.’” The countries of the South Pacific established a NWFZ in 1985, followed by Southeast Asia in 1995, Africa in 1996, and Central Asia in 2006. Additional treaties have designated outer space and the ocean floor as additional NWFZs. 

U.S. President Lyndon B. Johnson looks on as Secretary of State Dean Rusk signs the Treaty on the Nonproliferation of Nuclear Weapons on July 1, 1968.

Corbis via Getty Images

First International Treaty to Prevent Spread of Nuclear Weapons Is Signed

In June 1968, the UN General Assembly adopted a resolution endorsing the draft text of the Treaty on the Nonproliferation of Nuclear Weapons, and countries began signing the treaty. Under this landmark international agreement, countries without nuclear weapons agreed to never obtain them; they can, however, use atomic energy peacefully. The five countries with nuclear weapons at the time—China, France, the Soviet Union, the United Kingdom, and the United States—all joined the treaty, making a commitment to eventually disarm, but none has yet.

U.S. President Richard Nixon and Soviet leader Leonid Brezhnev sign the Interim Strategic Arms Limitations Treaty (SALT) agreement in the Kremlin in Moscow on May 26, 1972.

Richard Nixon Presidential Library and Museum

SALT I Treaty Is Signed

U.S. President Richard M. Nixon and the Soviet leader Leonid Brezhnev signed the interim Strategic Arms Limitations Talks agreement (SALT I), the first agreement between the United States and the Soviet Union to limit their nuclear arsenals during the Cold War. This agreement marked a victory for nonproliferation ten years after the Cuban missile crisis between the nuclear-armed rivals. SALT II, signed seven years after SALT I, by Brezhnev and U.S. President Jimmy Carter, further limited nuclear capabilities.

An intermediate range ballistic missile on a launcher during a parade in New Delhi on January 26, 2004.

Antônio Milena/Agência Brasil under CC BY 3.0 BR

India Joins the Nuclear Club

India conducted its first nuclear test, code-named Smiling Buddha, in May 1974. Although the country’s government then denied that it was pursuing a nuclear weapons program and claimed the explosion was for peaceful purposes, India now sees its nuclear program as central to its security and image as an emerging world power. This was the first time a country beyond the original five NPT-recognized nuclear-armed states had tested a nuclear bomb. Neighbor and rival Pakistan tested its first nuclear weapon in 1998.

End of the Cold War Improves Nonproliferation Efforts

During the Cold War, a period defined by tensions between the United States and the Soviet Union, the threat of nuclear war was always present. But after the Soviet Union collapsed in 1991 and the Cold War ended, real progress was made to strengthen the Nuclear Nonproliferation Treaty, get former Soviet countries to join the treaty, and stop further use of nuclear weapons.

A Ukrainian army officer looks at the destroyed SS-24 missile silo near the town of Pervomaisk in Ukraine's Nikolayev region on October 30, 2001.

Gleb Garanich/Reuters

U.S. Secretary of State James Baker, right, and Russian Foreign Minister Andrei Kozyrev sign a protocol to the Strategic Arms Reduction Treaty in Lisbon on May 23, 1992.

Fernando Ricardo/AP

Belarus, Kazakhstan, and Ukraine Give Up Nuclear Weapons

After the fall of the Soviet Union, three of its former territories—Belarus, Kazakhstan, and Ukraine—were left in possession of nuclear weapons. An international agreement among the three former territories and Russia required that all nuclear weapons within the territories either be destroyed or transferred to Russia for destruction. The three former Soviet republics also agreed to join the Nuclear Nonproliferation Treaty as “non-nuclear-weapon” countries and, along with South Africa, are the only countries to give up their nuclear arsenals.

A nuclear bomb detonates at the Mururoa atoll, French Polynesia in 1971.

AFP via AP

The Comprehensive Nuclear Test Ban Treaty Opens for Signatures

After two years of negotiations, the Comprehensive Nuclear Test Ban Treaty (CTBT) opened for signature at the United Nations. The treaty banned nuclear explosions of any kind, including for weapons tests. But the CTBT is not yet legally binding because not all the required countries—including China, India, Pakistan, and the United States—have signed or ratified it in their home countries. Even so, most countries with nuclear weapons—including the Soviet Union and later Russia, the United Kingdom, and the United States—have not conducted nuclear tests since the early 1990s.

The flag of the International Atomic Energy Agency (IAEA) flies in front of the Vienna headquarters.

Heinz-Peter Bader/Reuters

The IAEA’s Model Additional Protocol is Introduced

Part of the International Atomic Energy Agency’s mandate is to monitor whether countries are using nuclear technology for peaceful purposes, like energy generation, rather than weapons. This monitoring involves inspections of nuclear facilities and power plants. After the 1990–91 Gulf War, it was discovered that Iraq had pursued an undeclared nuclear weapons program despite being subject to IAEA inspections. In response, the IAEA’s board of governors approved the Model Additional Protocol, which gave the agency further access to information and nuclear sites. Although the protocol is an optional agreement, it is now implemented in 136 countries and the European Atomic Energy Community, strengthening the IAEA’s inspection capabilities.

Progress and Threats

Though many countries have worked to limit or eliminate nuclear weapons, threats remain from countries that continue to build arsenals, do not plan to completely disarm, or do not follow safety standards for nuclear material. Containing nuclear weapons and preventing nuclear war remains one of the greatest challenges facing world leaders today.

Russian President Vladimir Putin addresses high-ranking officials in Moscow, Russia on March 1, 2018. In his speech, Putin said Russia had tested "invincible" nuclear weapons that cannot be intercepted.

Maxim Shemetov/Reuters

A missile on a military vehicle during a parade in Pyongyang on July 27, 2013.

Jason Lee/Reuters

North Korea Withdraws From the Nonproliferation Treaty

North Korea announced its withdrawal from the Nuclear Nonproliferation Treaty (NPT), stating: “We can no longer remain bound to the NPT, allowing the country’s security and the dignity of our nation to be infringed upon.” Months earlier, the United States had announced that North Korean officials had admitted to enriching uranium for nuclear weapons. North Korea denied the claim but then reopened nuclear facilities it had previously shut down and ordered IAEA inspectors out of the country. Almost four years later, the North Korean government announced that it had completed a nuclear test, becoming the eighth country in history to do so.

Envoys to the six party talks on North Korea's nuclear issue shake hands at the beginning of another round of the talks in Beijing on July 10, 2008.

Greg Baker/Reuters

North Korea Walks Out of Six Party Talks

Negotiations among China, Japan, North Korea, Russia, South Korea, and the United States to find a peaceful resolution to North Korea’s nuclear weapons program fell apart after the UN Security Council condemned a North Korean test launch of a rocket, which it had disguised as part of its civilian space program. The negotiations, known as the Six Party Talks, had lasted six years but failed to reach a resolution. North Korea remains one of the most unstable nuclear powers today.

U.S. Secretary of State John Kerry talks with Iranian Foreign Minister Mohammad Javad Zarif in Vienna on January 16, 2016, after the International Atomic Energy Agency verified that Iran has met all conditions under the nuclear deal.

Kevin Lamarque/Pool via AP

World Powers Reach a Nuclear Agreement With Iran

In 2002, U.S. officials claimed that Iran had embarked on a nuclear weapons program. U.S. researchers published satellite photographs of what they identified as a large uranium enrichment plant and a heavy water plant, crucial equipment in the production of nuclear weapons. The United States and its allies asserted that Iran intended to build a nuclear weapon, despite the country’s denials and the IAEA’s statement that Iran had not violated its commitments. In 2015, the United States, China, France, Germany, Russia, the United Kingdom, and the European Union reached a nuclear agreement with Iran after years of negotiation, known as the Joint Comprehensive Plan of Action (JCPOA). After Iran agreed to limit its nuclear program and subject its nuclear facilities to much stricter monitoring than ordinary International Atomic Energy Agency safeguards, the United States and others relaxed sanctions on Iran’s economy. But in 2018, U.S. President Donald J. Trump announced that the United States would withdraw from the deal and reimposed sanctions on Iran.

A model of the first Chinese H-bomb (H639-23) is displayed in Beijing on September 21, 2009, at an exhibition showcasing the achievements China has made in the past six decades.

Jason Lee/Reuters

The United Nations Adopts Nuclear Weapons Ban Treaty

At the United Nations, 122 countries adopted the Treaty on the Prohibition of Nuclear Weapons, the first legally binding treaty for nuclear disarmament in twenty years. Countries that signed the treaty, which builds on the provisions of the Nuclear Nonproliferation Treaty and the Comprehensive Test Ban Treaty, see it as an important step toward the elimination of nuclear weapons. However, countries that already have nuclear weapons did not sign, so it remains to be seen how effective the treaty will be.

Referenced Module