For Syria, the country's chemical weapons program dates back more than 45 years ago when the country received help from Egypt before the 1973 Arab-Israeli War. By the 1980s, Syria was making its own chemical weapons and had several delivery systems to unleash them.
Defense experts believe North Korea, Iran and Russia played a role in the Syrian regime's offensive chemical weapons program, too.
Several chemical weapons attacks have been reported during the Syrian civil war, including the 2013 sarin nerve gas attack in the Ghouta district of Damascus. The Ghouta attack killed more than 1,400 people, according to U.S. government estimates.
Faced with international pressure, in 2013 Syria agreed to disarmament, and joined the Chemical Weapons Convention. An international group removed large stockpiles of chemical agents from Syria — but that didn't stop the regime from continuing its chemical weapons program.
Kimball, of the Arms Control Association, said precursor chemicals and agents — primarily sulfur mustard and sarin gas, along with related equipment — were removed from Syria under the 2013 agreement brokered by Russia and the United States.
Regardless, he said the regime reverted back cholorine-type barrel bomb attacks since the chlorine compound is widely available as an industrial chemical.
And the Syrians apparently didn't give up all the nerve agent sarin or made new batches since they were blamed for an attack using the nerve gas on civilians in April 2017. In response for that attack, Trump ordered a strike that launched 59 Tomahawk cruise missiles at a Syrian military airfield.