The United Nations prohibits the use of chemical weapons in so-called civilised warfare, because of the agonising suffering they cause.
So the recent use of a chemical weapon in attack on UK soil, allegedly by Russia, has not only shocked the world, but also driven a wedge between two of the world's most powerful nations.
The nerve agent used in the attack on Russian spy Sergei Skripal and his daughter Yulia, is one of a group known as Novichok.
It’s one of the most deadly chemical weapons ever created, and ingesting it causes the heart to slow down, restricts the airways and leads to death by asphyxiation.
Both Sergei Skripal and his daughter Yulia remain in a critical condition in hospital.
Sadly, the use of chemical weapons is becoming far more frequent, as new deadly nerve agents are created in the laboratory.
The first chemical weapons were used over a century ago during World War I.
Here’s a brief history of their use:
- 1914 - 18
Despite poisonous gas having been banned by the Hague Convention in 1907, most warring countries used various forms of deadly gas throughout World War I.
The most common types were chlorine and mustard gas, and it's thought that chemical weapons killed 90,000 soldiers and seriously wounded another 1 million. It's estimated around 500 Kiwi soldiers were killed outright by exposure to poison gas, while at least 5000 suffered life-long effects.
The Geneva protocol makes it illegal to use chemical weapons in war - but does not ban its production or stockpile.
The Italian army uses mustard gas during its invasion of Ethiopia.
- 1937 - 45
Japan uses cyanide and mustard gas against the Chinese military and civilians during the Sino-Japanese war.
- 1939 - 1945
Nazi Germany uses the chemical weapon Zyklon B to kill at least 6 million Jews in its European extermination camps. The warring countries did not use any chemical weapons during combat.
- 1947 - 91
Both the United States and the Soviet Union build up massive amounts of chemical weapons during the Cold War but none are used in combat. It's estimated these huge stockpiles could have killed most of the humans on the planet.
- 1980 - 88
Iraqi forces use the deadly nerve agents sarin and tabun against Iranian forces and civilians during the Iran-Iraq war.
Iraqi forces controlled by Saddam Hussein use sarin and mustard gas against the population of the Kurdish city of Halabja in northern Iraq. It's estimated up to 5000 people were killed.
A Japanese cult known as Aum Shinrikyo unleashes a homemade variant of the nerve agent sarin on the Tokyo subway. 21 people are killed and another 1000 affected.
The Syrian government uses sarin on the population of Damascus, killing 1400.
The Syrian government drops chlorine gas from helicopters on so-called rebel forces in the village of Kfar Zeita. Many civilians are among the dead.
North Korean leader Kim Jong-un's half brother Kim Jong-nam is killed by the deadly XV nerve agent in Malaysia. XV is thought to be 100 times more powerful than the sarin nerve agent. Two women attacked Jong-nam at Kuala Lumpur International Airport. It's widely believed his death was ordered by the North Korean government.
The Syrian government launches an airstrike using sarin against so-called rebels in the Idlib province - 70 people, including many small children are killed or affected. The attack prompts US President Donald Trump to launch an airstrike from a US aircraft carrier against the Syrian airbase thought responsible.
A deadly new nerve agent, Novichok, is used to poison Russian spy Sergei Skripal and his daughter Yulia in the small UK city of Salisbury. The chemical weapon is thought to have originated in Russia.
- New Zealand warned: Russian nerve gas attack 'could happen anywhere'
- Syria 'clearly' still using chemical weapons - US
- Tensions rise between Malaysia and North Korea over Kim Jong Nam killing