Chemical warfare
Efforts to eradicate chemical weapons: August 27, 1874: The Brussels Declaration Concerning the Laws and Customs of War is signed, specifically forbidding the "employment of poison or poisoned weapons", although the treaty was not adopted by any nation whatsoever and it never went into effect.
September 4, 1900: The First Hague Convention, which includes a declaration banning the "use of projectiles the object of which is the diffusion of asphyxiating or deleterious gases," enters into force.
January 26, 1910: The Second Hague Convention enters into force, prohibiting the use of "poison or poisoned weapons" in warfare.
February 6, 1922: After World War I, the Washington Arms Conference Treaty prohibited the use of asphyxiating, poisonous or other gases. It was signed by the United States, Britain, Japan, France, and Italy, but France objected to other provisions in the treaty and it never went into effect.
February 8, 1928: The Geneva Protocol enters into force, prohibiting the use of "asphyxiating, poisonous or other gases, and of all analogous liquids, materials or devices" and "bacteriological methods of warfare".

Chemical weapon proliferation: Despite numerous efforts to reduce or eliminate them, some nations continue to research and/or stockpile chemical warfare agents.
In 1997, future US Vice President Dick Cheney opposed the signing ratification of a treaty banning the use of chemical weapons, a recently unearthed letter shows. In a letter dated April 8, 1997, then Halliburton-CEO Cheney told Sen. Jesse Helms, the chairman of the Senate Foreign Relations Committee, that it would be a mistake for America to join the convention. "Those nations most likely to comply with the Chemical Weapons Convention are not likely to ever constitute a military threat to the United States. The governments we should be concerned about are likely to cheat on the CWC, even if they do participate," reads the letter, published by the Federation of American Scientists.
The CWC was ratified by the Senate that same month. In the following years, Albania, Libya, Russia, the United States, and India declared over 71,000 metric tons of chemical weapon stockpiles, and destroyed a third of them. Under the terms of the agreement, the United States and Russia agreed to eliminate the rest of their supplies of chemical weapons by 2012, but ended up taking far longer to do so as shown in the previous and following section of this article.

Chemical weapons destruction: 

India: In June 1997, India declared that it had a stockpile of 1044 tons of sulphur mustard in its possession. India's declaration of its stockpile came after its entry into the Chemical Weapons Convention, that created the Organisation for the Prohibition of Chemical Weapons, and on January 14, 1993, India became one of the original signatories to the Chemical Weapons Convention. By 2005, from among six nations that had declared their possession of chemical weapons, India was the only country to meet its deadline for chemical weapons destruction and for inspection of its facilities by the Organisation for the Prohibition of Chemical Weapons. By 2006, India had destroyed more than 75 percent of its chemical weapons and material stockpile and was granted an extension to complete a 100 percent destruction of its stocks by April 2009. On May 14, 2009, India informed the United Nations that it has completely destroyed its stockpile of chemical weapons.

Iraq: The Director-General of the Organisation for the Prohibition of Chemical Weapons, Ambassador Rogelio Pfirter, welcomed Iraq's decision to join the OPCW as a significant step to strengthening global and regional efforts to prevent the spread and use of chemical weapons. The OPCW announced "The government of Iraq has deposited its instrument of accession to the Chemical Weapons Convention with the Secretary General of the United Nations and within 30 days, on 12 February 2009, will become the 186th State Party to the Convention". Iraq has also declared stockpiles of chemical weapons, and because of their recent accession is the only State Party exempted from the destruction time-line.

Japan: During the Second Sino-Japanese War (1937–1945) Japan stored chemical weapons on the territory of mainland China. The weapon stock mostly containing sulfur mustard-lewisite mixture. The weapons are classified as abandoned chemical weapons under the Chemical Weapons Convention, and from September 2010 Japan has started their destruction in Nanjing using mobile destruction facilities in order to do so.

Russia: Russia signed into the Chemical Weapons Convention on January 13, 1993, and ratified it on November 5, 1995. Declaring an arsenal of 39,967 tons of chemical weapons in 1997, by far the largest arsenal, consisting of blister agents: Lewisite, Sulfur mustard, Lewisite-mustard mix, and nerve agents: Sarin, Soman, and VX. Russia met its treaty obligations by destroying 1 percent of its chemical agents by the 2002 deadline set out by the Chemical Weapons Convention, but requested an extension on the deadlines of 2004 and 2007 due to technical, financial, and environmental challenges of chemical disposal. Since, Russia has received help from other countries such as Canada which donated C$100,000, plus a further C$100,000 already donated, to the Russian Chemical Weapons Destruction Program. This money will be used to complete work at Shchuch'ye and support the construction of a chemical weapons destruction facility at Kizner (Russia), where the destruction of nearly 5,700 tons of nerve agent, stored in approximately 2 million artillery shells and munitions, will be undertaken. Canadian funds are also being used for the operation of a Green Cross Public Outreach Office, to keep the civilian population informed on the progress made in chemical weapons destruction activities.
As of July 2011, Russia has destroyed 48 percent (18,241 tons) of its stockpile at destruction facilities located in Gorny (Saratov Oblast) and Kambarka (Udmurt Republic) – where operations have finished – and Schuch'ye (Kurgan Oblast), Maradykovsky (Kirov Oblast), Leonidovka (Penza Oblast) whilst installations are under construction in Pochep (Bryansk Oblast) and Kizner (Udmurt Republic). As August 2013, 76 percent  (30,500 tons) were destroyed, and Russia leaves the Cooperative Threat Reduction (CTR) Program, which partially funded chemical weapons destruction.
In September 2017, OPCW announced that Russia had destroyed its entire chemical weapons stockpile.

United States: On November 25, 1969, President Richard Nixon unilaterally renounced the offensive use of biological and toxic weapons, but the U.S. continued to maintain an offensive chemical weapons program.
From May 1964 to the early 1970s the U.S. participated in Operation CHASE, a United States Department of Defense program that aimed to dispose of chemical weapons by sinking ships laden with the weapons in the deep Atlantic. After the Marine Protection, Research, and Sanctuaries Act of 1972, Operation Chase was scrapped and safer disposal methods for chemical weapons were researched, with the U.S. destroying several thousand tons of sulfur mustard by incineration at the Rocky Mountain Arsenal, and nearly 4,200 tons of nerve agent by chemical neutralisation at Tooele Army Depot.
The U.S. began stockpile reductions in the 1980s with the removal of outdated munitions and destroying its entire stock of 3-Quinuclidinyl benzilate (BZ or Agent 15) at the beginning of 1988. In June 1990 the Johnston Atoll Chemical Agent Disposal System began destruction of chemical agents stored on the Johnston Atoll in the Pacific, seven years before the Chemical Weapons Treaty came into effect. In 1986 President Ronald Reagan made an agreement with German Chancellor Helmut Kohl to remove the U.S. stockpile of chemical weapons from Germany. In 1990, as part of Operation Steel Box, two ships were loaded with over 100,000 shells containing Sarin and VX were taken from the U.S. Army weapons storage depots such as Miesau and then-classified FSTS (Forward Storage / Transportation Sites) and transported from Bremerhaven, Germany to Johnston Atoll in the Pacific, a 46-day nonstop journey.
In the 1980s, Congress, at the urging of the Reagan administration, Congress provided funding for the manufacture of binary chemical weapons (sarin artillery shells) from 1987 until 1990, but this was halted after the U.S. and the Soviet Union entered into a bilateral agreement in June 1990. In the 1990 agreement, the U.S. and Soviet Union agreed to begin destroying their chemical weapons stockpiles before 1993 and to reduce them to no more than 5,000 agent tons each by the end of 2002. The agreement also provided for exchanges of data and inspections of sites to verify destruction. Following the collapse of the Soviet Union, the U.S.'s Nunn–Lugar Cooperative Threat Reduction program helped eliminate some of the chemical, biological and nuclear stockpiles of the former Soviet Union.
The United Nations Conference on Disarmament in Geneva in 1980 led to the development of the Chemical Weapons Convention (CWC), a multilateral treaty that prohibited the development, production, stockpiling, and use of chemical weapons, and required the elimination of existing stockpiles.