Global leaders are condemning Syria for what President Trump calls its “barbaric” chemical weapon attack on April 4 that killed scores of innocent civilians.

“There can be no dispute that Syria used banned chemical weapons, violated its obligations under the Chemical Weapons Convention and ignored the urging of the U.N. Security Council,” President Trump said.

The United Nations has called the use of chemical weapons “unacceptable.”

Syria four years ago acceded to the Chemical Weapons Convention, a treaty that bans use of chemical weapons and requires nations to destroy their stockpiles with international verification. The treaty seeks “for the sake of all mankind” to remove any possibility of the use of chemical weapons.

What are chemical weapons?

Illustration of chemistry beaker with bomb inside (State Dept./Doug Thompson)They are toxic chemicals used to kill or harm a target. They can be delivered by missiles, bombs, artillery shells, mines or other means. They can cause victims to choke, suffer seizures and, in lethal doses, die within five minutes.

Some, such as mustard gas, have delayed effects. They can be liquids, gases, aerosols, vapors or dust. Among the most rapid and deadly are nerve agents, such as sarin and VX.

What is the Chemical Weapons Convention?

Illustration of chemistry beaker with image of globe inside and two olive branches alongside (State Dept./Doug Thompson)Several international agreements, including the Geneva Protocol of 1925, have sought to end the use of chemical and biological weapons in war. The Chemical Weapons Convention is the most far-reaching. (The Geneva Protocol outlawed the use but not the possession of chemical weapons.)

The convention is a landmark agreement that was negotiated in Paris in 1993 and took effect in 1997 when the 65th country ratified it. Now 192 countries are party to the convention, including Syria, since 2013, and — the two most recent — Burma and Angola, since 2015.

The convention bans chemical weapons and requires their destruction and the elimination of production facilities.

Parties to the convention must declare the extent of their chemical weapons stockpiles and destroy them in a specified time period. The convention establishes a comprehensive inspection regime of both government and private chemical facilities to verify that no new chemical weapons are being produced. Signatories cannot assist or encourage others to use chemical weapons.

How does the convention work and who enforces it?

Illustration of white dove perched on top of corked chemistry beaker with bomb inside (State Dept./Doug Thompson)The Organisation for the Prohibition of Chemical Weapons, based in The Hague, Netherlands, sends out inspectors to verify that countries meet their treaty obligations.

The organization won the 2013 Nobel Peace Prize for its work.

The U.S. Department of State’s Bureau of Arms Control, Verification and Compliance oversees U.S. implementation of the convention.

What difference has it made?

Illustration of white dove placing olive branch in front of chemistry beaker with globe inside (State Dept./Doug Thompson)Ninety-four percent of member states’ chemical weapons stockpiles have been verified as destroyed, says organization Director-General Ahmet Üzümcü.

The organization has conducted 6,327 inspections and destroyed about 72,300 metric tons of banned stockpiles. Some 7.4 million munitions have been destroyed. Member states have destroyed or converted about 90 chemical weapon production facilities.

It continues to monitor the scheduled destruction of members’ remaining stockpiles.

During World War I, chlorine, mustard gas and other toxic chemicals were widely used in battle, killing upwards of 100,000 people. While the world is not free of war, combatants in the 21st century generally have not resorted to chemicals as a weapon of mass destruction.

Syria’s civil war is an exception.

Illustrations by Doug Thompson/State Dept.