Hiroshima, Japan | Here is a timeline of the development of nuclear weapons, as US President Barack Obama pays a historic visit to the Japanese city of Hiroshima.

Hiroshima and Nagasaki are the only two cities to suffer an atomic bombing, by US planes in August 1945.

- June 1942: The United States launches the top-secret "Manhattan Project" to build an atomic bomb before the Nazis do. More than $2 billion is spent to achieve that goal.

- July 1945: The early morning "Trinity" test takes place in New Mexico, marking the dawn of the nuclear age.

- August 1945:  On August 6, a US bomber drops an atomic bomb built with uranium on Hiroshima, killing 140,000 people and wounding tens of thousands.

Three days later, a second atomic bomb with plutonium fuel smashes Nagasaki, killing 70,000 people.  On the 15th, Japan surrenders.

- August 1949: Four years after Hiroshima and Nagasaki are destroyed, the Soviet Union successfully tests its own atomic bomb in Kazakhstan. Britain becomes the world's third nuclear power with an A-bomb test in Australia in October 1952.

- November 1952: The US tests its first hydrogen, or thermonuclear bomb (H-bomb), in the Pacific. It is almost 700 times more powerful than an atomic bomb. The Soviet Union tests its first H-bomb in 1953, followed by the British in 1957.

France then tests an A-bomb in February 1960, as does China in October 1964. Both countries follow suit a few years later with H-bomb tests.

- February 1967: The Tlatelolco treaty declares Latin America a nuclear-free zone. It is followed by other treaties that cover the Pacific, South-East Asia, and Africa.

- July 1968: Britain, the Soviet Union and the United States sign the Non-Proliferation Treaty (NPT) which takes effect in March 1970.

- May 1998: India and Pakistan become nuclear powers.

- October 2006: North Korea, which withdrew from the NPT in 2003, detonates an atomic device, and follows with three more tests since then. North Korea is also developing ballistic missile technology.

In December 2006, Israeli authorities let it be known they possess nuclear weapons, and the country is also developing long-range missiles.

- April 2010: Russia and the US sign a second Strategic Arms Reduction Talks (START) agreement to replace one signed in 1991. It calls for a significant reduction in the nuclear weapons arsenals of both countries. Britain is also reducing its stocks, while France and Israel are believed to be maintaining a stable level. According to the Federation of American Scientists, China, India, North Korea and Pakistan are building their inventories of warheads.

- July 2015: An agreement between Iran and major powers is signed with the aim of ensuring that Iran's nuclear programme remains limited to civilian purposes. In exchange, international sanctions against Iran are lifted.  

According to the Stockholm International Peace Research Institute (SIPRI), the number of nuclear warheads has decreased, but the nine known nuclear powers continue to develop more sophisticated nuclear weapons. 

In early 2015, SIPRI estimated the total number of nuclear warheads worldwide at 15,850 of which 4,300 are considered operational. In 2010 the numbers were estimated at 22,600 and 7,650 respectively. 

Russia and the United States account for most of the reduction, but they still hold about 7,500 for the former and 7,260 for the latter, or 90 percent of the total.