Over the course of the Syrian conflict, chemical warfare has caused a high number of casualties. But it's not the first time mankind has employed this controversial art of war.
Chemical weapons are often a "red line" in policy, if not practice, for many nations.
Here's a brief timeline of key moments in the history of chemical warfare:
But the debate over the various types of chemical weapons, and the international commitment to ban their production, possession, and development is constantly being reopened as new weapons are developed.
Which chemicals are commonly used as weapons of war?
The Organisation for the Prohibition of Chemical Weapons is a UN body that has had oversight of chemical weapons in Syria. In 2014, the group claimed that the removal of chemical weapons from the Syrian regime was complete. But the regime continued to use them.
Who has signed up to the Chemical Weapons Convention?