The Empire of Japan extensively used and researched chemical and biological weapons (CBW) during the Second Sino-Japanese War as part of Japanese war crimes. Japan is the only country ever attacked with nuclear weapons, by the United States' 1945 atomic bombings of Hiroshima and Nagasaki at the end of World War II. During the Cold War, the United States stationed chemical and nuclear weapons in Japan from the early 1950s to early 1970s. Postwar Japan ratified the Nuclear Non-Proliferation Treaty, Biological Weapons Convention, and Chemical Weapons Convention. The US provides a nuclear umbrella to Japan.

Beginning in the mid-1930s, Japan conducted numerous attempts to acquire, develop and use weapons of mass destruction. Japan's biological warfare is estimated to have killed between 200,000 and 500,000 people in China. Biological warfare units, led by Unit 731, operated across the Japanese colonial empire, dispersing anthrax, cholera, dysentery, typhoid, plague, and others. In the Japanese puppet state of Manchukuo, Unit 731 and Unit 516 were centers of CBW research, including human experimentation. CBW was used at the Battle of Changde, the Battle of Wuhan, the Ningbo plague attack, and elsewhere. The Imperial Japanese Navy planned a weaponized plague attack against California. Unit 731 members provided information to the Soviet and United States biological weapons programs, twelve of whom were found guilty in the Khabarovsk war crimes trials. From 1940 to 1945, Japan conducted a nuclear weapons program which did not progress past laboratory experiments. The US covered up Japan's biological warfare, and the Japanese government first acknowledged the program in 2002.

The US atomic bombings, the culmination of the Manhattan Project, devastated Hiroshima and Nagasaki, killing between 150,000 and 246,000 people within four months. The ethics and role of the attacks in the unconditional surrender of Japan and end of the Pacific War are the subject of continued debate. The attacks had a strong influence on Cold War's nuclear arms race and in popular culture.

From 1954 until 1972, the US stationed a range of strategic and tactical nuclear weapons, especially in the US-administered Ryukyu Islands. These included nuclear tactical ballistic missiles, cruise missiles, artillery, anti-air, anti-submarine weapons, and strategic bombers. Following the 1954 US Castle Bravo thermonuclear test, one Japanese fisherman died when all 23 crew aboard the Daigo Fukuryū Maru suffered acute radiation syndrome. The US stockpiled thousands of tons of chemical weapons in Okinawa, including mustard gas, sarin, and VX, which were removed in 1971.

Article 9 of the Constitution of Japan has been interpreted to prohibit the country from weapons of mass destruction and long-range delivery systems. Under the Three Non-Nuclear Principles established in 1971, Japan rejects possessing, producing, and introducing into its territory nuclear weapons. However, postwar Japan is an exemplar of nuclear latency: possessing the materials and technical capacity for an indigenous nuclear weapons program. This includes an advanced civil nuclear program, reprocessing, and stockpile of plutonium. Some 21st century politicians have supported a role for US nuclear weapons in Japan.

The Japanese religious cult Aum Shinrikiyo carried out the first chemical terrorist attacks to use nerve agents. Members perpetrated at least 10 such attacks, notably using sarin in Matsumoto in 1994 and on the Tokyo subway in 1995, as well as some of the first attacks to use VX.Pamela Zurer. "Japanese cult used VX to slay member". Chemical and Engineering News. 1998, Vol 76 (no. 35), 7. The group also unsuccessfully attempted bioterrorist attacks with anthrax and botulinum toxin, which remains the most extensive non-state biological weapons program to date.

Japanese possession of weapons of mass destruction Biological weapons

The Empire of Japan's use of biological weapons in the Second Sino-Japanese War is estimated to have killed between 200,000 and 500,000 people, almost entirely in China.Lockwood, Jeffrey A. "Six-legged soldiers ", The Scientist, October 24, 2008, accessed December 23, 2008.Novick, Lloyd and Marr, John S. Public Health Issues Disaster Preparedness, (Google Books), Jones & Bartlett Publishers, 2001, p. 87, (). Following research and production at its biological warfare units led by Unit 731, Japanese forces dispersed anthrax, cholera, dysentry, glanders, typhoid, and plague via airplane-dropped bombs containing infected fleas, a form of entomological warfare. Shirō Ishii, the leader of Unit 731, carried out two major anti-civilian campaigns. The first from 1940 to 1942 dispersed plague-infected fleas in port cities in northern China. The second in 1943 used anthrax and glanders against villages southwest of Shanghai.

Attacks in China Japan became interested in obtaining biological weapons during the early 1930s. Following the international ban on germ warfare in interstate conflicts by the 1925 Geneva Protocol, Japan reasoned that disease epidemics make effective weapons. Japan developed new methods of biological warfare (BW) and used them on a large scale in China. During the Sino-Japanese War and World War II, Unit 731 and other Special Research Units of the Imperial Japanese Army conducted human experimentation on thousands, mostly Chinese, Korean, Russian, and other nationalities, mainly across Asia (Taiwanese, Vietnamese, Cambodians, Filipinos, etc.), as well as some American, British and Dutch POWs and Japanese criminals.AII POW-MIA Unit 731

In military campaigns, the Japanese army used biological weapons on Chinese soldiers and civilians. Shirō Ishii, the leader of Unit 731, carried out two major anti-civilian campaigns. The first from 1940 to 1942 dispersed plague-infected fleas in port cities in northern China. The second in 1943 used anthrax and glanders against villages southwest of Shanghai. This was in retaliation for their assistance to US pilots of the 1942 Doolittle Raid, the first US air raid against Honshu.

Unit 731 used plague-infected fleas and flies covered with cholera to infect the population in China. The Japanese military dispersed insects by spraying them from low-flying airplanes and dropping ceramic bombs they had developed that were filled with mixtures containing insects and diseases that could affect humans, animals, and crops.Lockwood, Jeffrey A. "Bug Bomb", The Boston Globe, October 21, 2007, accessed December 23, 2008. Recent additional firsthand accounts testify the Japanese infected civilians through the distribution of plague-infested foodstuffs, such as dumplings and vegetables. During the Changde chemical weapon attacks, the Japanese also employed biological warfare by intentionally spreading infected fleas. In Zhejiang, anthrax, cholera, dysentery, and typhoid were employed. Harbin also suffered Japanese biological attacks. Other battles include the Kaimingye germ weapon attack in Ningbo.

Planned attacks against U.S. forces and mainland Japan sent a submarine with unspecified biological weapons early in 1944 to defend the island of Saipan from American invasion; however the submarine was sunk.

Another attack against American troops with biological weapons was planned for the invasion of Iwo Jima. The plan involved towing gliders laden with pathogens over the American lines. However, this plan never took shape. Had it succeeded, thousands of American soldiers and marines may have died, and the operation as a whole may very well have failed.

Japan's biowarfare experts had hoped to launch biological attacks on the U.S. in 1944 with balloon bombs filled with bubonic plague, anthrax, rinderpest, and smut fungus. A 1945-planned kamikaze attack on San Diego with I-400-class submarine aircraft carriers that would deploy Aichi M6As floatplanes and drop fleas infected with bubonic plague was code-named Operation Cherry Blossoms at Night. The plans were rejected by Hideki Tojo who feared similar retaliation by the United States.

Postwar legacy Japanese scientists from Unit 731 provided research information for the United States biological weapons program in order to escape war crimes and crimes against humanity charges. Researchers arrested by Soviet forces were tried at the December 1949 Khabarovsk war crimes trials; they were given light sentences to the Siberian labor camp, from 2 to 25 years, seemingly in exchange for the information they held.Harris, S.H. (2002) Factories of Death. Japanese Biological Warfare, 1932–1945, and the American Cover-up, revised ed. Routledge, New York. Bob Dohini, a former war crimes prosecution team lawyer, recently claimed that there was no mention of germ warfare in the investigation of Japanese war crimes. The fact was not well known until the 1980s. Japan's emperor was unable to be tried. Later revelations indicated his knowledge of the program.

Japan's employment of BW was largely viewed as ineffective, due to the absence of efficient production or delivery technology. The U.S. government provided a stipend to the Japanese BW military scientists and researches. Japanese biological warfare information provided to U.S. authorities after World War II remained a secret and was eventually returned to Japan.

Right-wing Japanese officials have claimed that no proof of Japan's wartime atrocities exists.

In August 2002, a Japanese court ended decades of official denials and acknowledged, for the first time, that Japan had used germ warfare in occupied China in the 1930s and 1940s. The court acknowledged the existence of Japan's biological warfare program but rejected the plaintiffs' demands for compensation, saying the issue was covered under postwar treaties. Following the court decision, Japanese officials announced that their government would send a delegation to China to excavate and remove hundreds of abandoned chemical weapons, including bombs, shells, and containers of mustard gas and other toxins left over from the Second World War.

Japan's biological warfare experts and scientists from WWII, were alleged to have assisted the US in employment of BW in the Korean War.Sheldon H. Harris (3 May 2002). Factories of Death: Japanese Biological Warfare, 1932-45, and the American Cover-up. Taylor & Francis.

Chemical weapons

Pre-Second Sino-Japanese War In 1927, the Imperial Japanese Army appropriate Ōkunoshima island, and constructed its primary chemical weapons production facility there. Its position in the Seto Inland Sea and small circumference of four kilometers, allowed for safety and secrecy, with the nearest city three kilometers away near Takehara, Hiroshima. In 1929, the Okunoshima site began producing tear and mustard gases. At peak capacity, Okunoshima produced per month approximately 200 tons of mustard gas, 50 tons of lewisite (also a blister agent), 80 tons of diphenylcyanoarsine (a vomiting agent), 50 tons of hydrogen cyanide (a blood agent), and 2.5 tons of chloroacetophenone (a more toxic riot control agent).

In the 1930 Musha Incident, Japan's army air corps in Taiwan ordered bombing runs over Musha to smoke out the rebels of the Seediq indigenous people, dropping mustard gas bombs in what was allegedly the first such use of chemical warfare in Asia.Eric Croddy, "China's Role in the Chemical and Biological Disarmament Regimes", The Nonproliferation Review Spring 2002: 16, <http://cns.miis.edu/npr/pdfs/91crod.pdf>, accessed 24 September 2011, p. 17.