Rebels seized control of Syria’s capital on Sunday, ending the reign of Bashar al-Assad, who infamously used chemical weapons to kill thousands of his own citizens during the country’s 13-year-long civil war.

Now, a militia group that once had ties to the Islamic State and al Qaeda, before renouncing them in 2016, is in control of Damascus and, presumably, what is left of Mr. al-Assad’s arsenal, prompting concern in the United States and elsewhere that the chemical weapons could fall into the wrong hands.

Here is what you need to know about Syria’s chemical weapons.

Mr. al-Assad’s use of chemical weapons on rebel groups and civilians was a defining feature in the early days of Syria’s civil war, so much so that in 2012, then U.S. President Barack Obama warned that their continued use would cross a “red line” that would justify U.S. military intervention.

Mr. al-Assad ignored the warning, and on Aug. 21, 2013, he launched a sarin gas attack on Ghouta, a suburb near Damascus, killing more than 1,400 civilians, including hundreds of children.

Under threat of U.S. retaliation, Mr. al-Assad agreed to a Russian-American deal to eliminate his country’s chemical weapons program — which until that time he had denied having — and to join an international treaty banning chemical weapons.

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