On Thursday night, the U.S. opened a third air campaign in Syria, targeting Bashar al-Assad’s air force. The U.S. has been bombing the Islamic State and select al Qaeda targets in Syria since 2014. But the Syrian regime, which is responsible for most of the civilian casualties in the country, was not hit with America’s air power until President Trump drew a new red line over Assad’s use of chemical weapons.

On Tuesday, Assad’s airmen dropped a chemical agent on citizens living in the town of Khan Shaykhun, which is in the northwestern province of Idlib. Trump expressed horror the following day. “It crossed a lot of lines for me. When you kill innocent children, innocent babies, little babies, with a chemical gas that is so lethal—people were shocked to hear what gas it was—that crosses many, many lines, beyond a red line, many, many lines,” Trump said, during a press conference alongside King Abdullah II of Jordan in the Rose Garden.