Physicists had known of the tremendous energy contained in the atom ever since Lord Rutherford's investigations of radioactivity early in the 20th century. It was not until 1938, however, that it became apparent that it might be possible to make an explosive weapon that would utilize this energy. Late that year the German scientists Otto Hahn and Fritz Strassmann succeeded in fissioning a small number of uranium atoms in the laboratory.

News of this achievement spread rapidly, and physicists in many nations realized that a “uranium bomb” might now be feasible. Europe at that time was on the brink of World War II, and many of Germany's scientists had fled that country because of the Nazi persecution of Jews. Among these displaced scientists was Albert Einstein, who was living in the United States. A group of scientists, upon hearing of the Hahn-Strassmann experiments, persuaded Einstein to write a letter to President Franklin D. Roosevelt.

Manhattan Project

In 1939, just months before the start of World War II, the physicists in the United States had become aware of the military applications of nuclear energy. They were worried about Nazi Germany who might develop a nuclear weapon. German-born physicist Albert Einstein's letter, to U.S. President Franklin D. Roosevelt, written in August, 1939, suggested that a nuclear bomb might be feasible, and that the United States should investigate the possibility of developing such a bomb. World War II began on Sept. 1, 1939. President Roosevelt appointed a committee to look into the matter, and in August, 1942, a program to develop a nuclear bomb was established under the code name “Manhattan Engineer District,” with Brigadier General Leslie R. Groves in charge. The program soon became known as the Manhattan Project.

In December, 1941, the United States had entered World War II, and many of its top scientists and engineers were assigned to the Manhattan Project. The first definite indication of ultimate success came on December 2, 1942. On that date a self-sustaining chain reaction was achieved with a primitive uranium reactor (or “pile”) set up under the west stands of Stagg Field at the University of Chicago.

The Italian-American physicist Enrico Fermi directed this operation, which ushered in the nuclear age. The enormous technical difficulties involved in isolating a sufficient quantity of uranium 235, which constitutes only 0.72 per cent of natural uranium, had to be overcome before this experiment could be attempted.

Early in 1943, a special laboratory to develop the nuclear bomb was set up at Los Alamos, New Mexico, with the physicist Robert Oppenheimer as director. On July 16, 1945, a 22-kiloton plutonium bomb was exploded on a steel tower in the desert near Alamogordo, New Mexico. It was an implosion-type fission device that convinced U.S. leaders that fission weapons could be built.

Just 21 days later, on August 6, 1945, a 13-kiloton uranium bomb was dropped on Hiroshima, Japan from an American B-29 aircraft. Three days later, a 22-kiloton plutonium bomb identical to the one tested at Alamogordo was dropped on Nagasaki, Japan, again from another American B-29 aircraft. The uranium used in the Hiroshima bomb was produced at Oak Ridge, Tennessee and the weapon was a gun-type fission bomb. The plutonium used in the Alamogordo and Nagasaki bombs was manufactured in a reactor at Hanford, Washington.

Postwar Projects

At the conclusion of World War II in the summer of 1945, many United States scientists and political leaders favored ending the manufacture of nuclear weapons for moral and other reasons. Production and testing continued, however, under the direction of the U.S. Atomic Energy Commission, which was established by Congress in 1946 to replace the Manhattan Engineer District.

A cold war had started between the Soviet Union and the U.S following the Second World War. In September, 1949, President Truman announced that the Soviet Union had exploded its first nuclear device. This news caused a considerable amount of concern in the United States, and the American nuclear weapons program was accelerated. Emphasis was placed on the development of a fusion bomb.

Some scientists thought it was technically impossible to develop a fusion bomb; others believed that the large amounts of radioactivity that it would produce could endanger humanity. However, a group led by the Hungarian-American physicist Edward Teller prevailed, and work on the bomb began in 1950. In November, 1952, during the Korean War, the United States detonated the world's first fusion weapon on Eniwetok Atoll in the Pacific. The Soviet Union exploded its first fusion bomb in 1955. During the mid 20th century, the Soviets built their first nuclear missiles equipped submarines. This was followed by the test launch of their first land-based intercontinental ballistic missile (ICBM) in 1957. The first U.S. ICBM came into being in 1959 which was immediately followed by the commissioning of their first submarine equipped with ballistic missile.

The United Kingdom exploded a fission bomb in 1952. In 1957 that country exploded its first hydrogen bomb. France became the fourth member of the “nuclear club” in 1960 by detonating a fission device. In 1964 China exploded a small uranium bomb, and in 1967 it detonated its first hydrogen bomb. France exploded its first fusion bomb in 1968. In 1974 India became a nuclear power by detonating a fission device.

Between 1989 and 1992 political developments in eastern Europe and the Soviet Union ended the Cold War. The focus was on reducing nuclear arsenals and properly controlling them. After the terrorist attacks of Sept. 11, 2001, a new threat from terrorists has taken shape who might gain access to nuclear arms, with the support of hostile nations.

Control of Nuclear Weapons

The strategies of deterrence and the limitations on the testing, numbers, and proliferation of nuclear weapons have been the main approaches to control nuclear weapons. The theories of deterrence may be offense-based or defense-based. According to the offense-based deterrence theory, possession of a strong nuclear force by two opposing nations best prevents nuclear war. A defender would retain enough weapons to devastate an aggressor even if attacked first. The theory was widely called the doctrine of mutually assured destruction (MAD).

As early as 1946 the United Nations had become concerned about the possible proliferation of nuclear weapons throughout the world as well as about the effects of radioactive fallout from uncontrolled test explosions. In 1958, the United States, the Soviet Union, and United Kingdom began an informal moratorium on testing. It was ended, however, by a Soviet test series that began in 1961.

After many more tests and repeated negotiation, the United States, the Soviet Union, and United Kingdom signed and ratified in 1963 a limited test-ban treaty, which was the first test limitation treaty. It forbade all tests except those conducted underground, which create little or no fallout. Open to all nations for signature, the treaty was signed by more than 100 countries. France and China did not sign it, however, and continued above-ground testing. ( 1963.)

In 1967 the United States, the Soviet Union, United Kingdom, and other nations signed a pact banning the use of nuclear weapons in outer space. The Nuclear Nonproliferation Treaty, intended to halt the spread of nuclear weapons to nonnuclear nations, went into effect in 1970 after being ratified by 43 nations.

Limiting proliferation involves preventing the spread of nuclear weapons to nations that do not possess them. The United Nations approved the Treaty on the Non-Proliferation of Nuclear Weapons in 1968. Since then, the treaty has been ratified by almost all countries.

Attempts to limit the number of U.S. and Soviet nuclear weapons began about 1970. In 1971, the Soviet Union and the United States signed a treaty providing for a system of consultations that would prevent nuclear accidents from turning into war. The two powers in 1972 signed SALT I, which included strict limits on each countrys defenses against nuclear missiles. This treaty consisted of two accords, the ABM treaty and the Interim Offensive Agreement. The ABM treaty limited the number of each nation's anti-ballistic missilesto one missile site and required that the site have no more than 100 missiles. The Interim Offensive Agreement limited for a five-year period the size of offensive ballistic missiles of each nation. United States and the Soviet Union agreed not to test explosive devices with yields above 150 kilotons in 1974. This agreement, contained in the Threshold Test Ban Treaty, which finally took effect in 1990, the year it, was agreed by both countries though both the nations had followed its guidelines long before its endorsement. SALT II was signed by the two powers in 1979, placing limits on the number of warheads, launch vehicles, and other elements in nuclear strength. Although the United States did not ratify the treaty, the country abided by its terms for eight years, when finally U.S. Senate refused to ratify the 1979 SALT II treaty after Soviet forces invaded Afghanistan.

According to the defense-based deterrence theory, a giant nuclear power will not attack unless sure that it can destroy opponents ability to launch a nuclear counterattack. However, the theory also says that only an effective defense against a powerful first strike will protect a defenders ability to retaliate.

Military experts believe that in cases of threat from a limited nuclear attack, an effective defense will either deter or, at least, bound the chief destruction. However, no country today has an effective defense. From 1983 to 1993, the United States carried out research on the Strategic Defense Initiative (SDI), a space-based system of defense against missiles. It was originally intended as a complete protection against nuclear attack but later aimed at protection against limited nuclear attacks only.

By 2000, people in the United States supported the idea of building a limited defense against attacks by few nuclear weapons which could be launched by terrorist or rogue nations, which are small or medium-sized nations that ignore international law and support terrorism. There were objections against this stating that developing such a national missile defense system would violate the ABM Treaty. After the Sept. 11, 2001, attacks on the United States, they withdrew from the ABM Treaty in June 2002 and increased funds on a defensive system. They also started developing a basic ground-and sea-based ballistic missile defense system for the west coast.

In 1987 the Soviet Union and the United States signed the Intermediate-range Nuclear Forces (INF) treaty. All Soviet and American ground-launched nuclear missiles with ranges of 310 to 3,420 miles (500 to 5,500 km) deployed in Europe were destroyed.

In 1990, the United States Senate ratified two treaties limiting underground nuclear explosions to 150 kilotons or less—the Threshold Test Ban Treaty, signed 16 years earlier, in 1974, and the Peaceful Nuclear Explosions Treaty, signed in 1976.

In 1991, the Soviet Union and the United States signed the Strategic Arms Reduction Treaty (START I), which U.S. President Ronald Reagan had initiated in 1982. It aimed at deep reductions in offensive strategic weapons that took approach in 1987, when Reagan and Soviet President Mikhail S. Gorbachev signed the Intermediate-Range Nuclear Forces (INF) Treaty. This treaty called for the elimination of all U.S. and Soviet ground-launched nuclear missiles with ranges of 500 to 5,500 kilometers (310 to 3,420 miles). It took effect in 1988. According to START, both sides agreed to steeply reduce their nuclear armaments. The Soviet Union broke up in late 1991, and in 1992 the United States and four successor nations of the Soviet Union that possessed nuclear weapons—Russia, Belarus, Kazakhstan, and Ukraine—signed a protocol to START I in May, 1992, that required Belarus, Kazakhstan, and Ukraine to eliminate their nuclear arsenals. The START I treaty came into force in 1994, when the last signatory, Ukraine, ratified it.

Efforts to limit the use of nuclear weapons took pace following the INF Treaty. Major political reforms in the Soviet Union and eastern Europe, including the fall of most Communist governments, led to reduced tensions and general arms reductions between 1989 and 1992. In 1990, the United States, the Soviet Union, along with 20 other nations signed the Treaty on Conventional Armed Forces in Europe (CFE) which took effect in 1992, and led to the destruction of a large numbers of tanks and other conventional weapons in Europe.

With much of the international tensions reduced, and the INF and CFE treaties in place, George H. W. Bush, U.S. president and Gorbachev, Soviet president signed the first treaty to significantly reduce existing numbers of strategic nuclear weapons in July 1991. The START I, aimed to reduce the number of U.S. and Soviet long-range nuclear missiles and bombers from 23,500 to 15,400 for both countries officially took effect in 1994. Soon in September 1991, United States announced the destruction of all its ground-based tactical nuclear weapons and many of those which were carried by ships and aircraft. The Soviet Union announced similar steps in the following month.

Meanwhile, in 1993 Russia and the United States signed START II, under the leadership of President George H. W. Bush and Russian President Boris Yeltsin, which called for further reductions in nuclear armaments, to between 6,000 and 7,000 for all of the countries combined. In 1995 representatives of 174 nations voted to renew the Nuclear Nonproliferation Treaty.

Ukraine, Belarus, and Kazakhstan had turned over all their nuclear weapons to Russia by late 1996. U.S. President George W. Bush and Russian President Vladimir Putin signed an agreement in 2002, which aimed at further cutting down their strategic nuclear arsenals to between 1,700 and 2,200 warheads for each country by 2012. This agreement, which was commonly known as the Treaty of Moscow, came into effect in 2003.

The United Nations sponsored the Geneva Conference on Disarmament in 1994. The conference drafted the Comprehensive Test Ban Treaty, which bars all nuclear tests. By mid-1998 it had 150 signatories including the United States and Russia. The Comprehensive Nuclear Test Ban Treaty was approved in 1996, by the United Nations, which would end all testing of nuclear weapons. Before it becomes effective, it has to be accepted by all countries having nuclear reactors, which are devices for producing nuclear energy. Till date, about three-fourths of those nations have signed the treaty.

START II was ratified by the United States Senate in 1996, but by late 1998 had yet to be ratified by the Russian Duma. In 1997 Presidents Clinton and Yeltsin signed an agreement to begin negotiations on START III. Non-proliferation efforts suffered a setback in 1998 when both India and Pakistan tested nuclear weapons. Neither country had signed the Comprehensive Test Ban Treaty, but began discussions with the UN after their tests.