• Thousands of rebel fighters in Douma agreed on Sunday to hand the area over to the government and be bused to an area outside the government’s control in the country’s north.
What we don’t know
• Whether the injuries were the result of a chemical weapon attack is not yet confirmed. The state news media in Syria denied that the government had used chemical weapons, and accused a rebel group of fabricating the videos to drum up international support. Russia and Iran, Syria’s allies, have also denied that Mr. Assad used chemical weapons. The United States and its allies believe chemical weapons were used.
• The United Nations has not determined responsibility for the attack. Rival resolutions from the United States and Russia on how to investigate the attack were defeated on Tuesday.
• The United States was still assessing the evidence of the attack on Wednesday and did not know which chemical was used, or whether it was launched by the Syrian government or forces supporting the government.
• The United States has not settled on a military response. On Wednesday, President Trump threatened a missile strike, but has not yet followed through and the specific strategy is unknown.
What recent history has taught us
• The use of chemical weapons, including sarin, chlorine, mustard gas and poison gas, in Syria has shown no signs of abating since it began in August 2013.
• An April 2017 attack that killed dozens of people in Khan Sheikhoun, in northern Syria, offered the first look at how the Trump administration would respond. President Trump ordered a military strike against the airfield where the weapons were launched, an approach meant to send a message that policy had changed from the Obama administration and that attacks would not be tolerated.
But the strike had little practical effect, with Syrians using the airfield again within 24 hours, according to the Syrian Observatory for Human Rights, a monitoring group.
• The U.S. has declined to intervene otherwise, even after several attacks with higher death tolls. Days before the weekend attack, Mr. Trump said he wanted to pull the United States out of Syria, a prospect dreaded by America’s regional allies and cheered by Russia.
What to watch for next
• The United States has three options for a military response, and none are particularly good. The approach of limited, punitive strikes, as the Trump administration opted for last year, has not changed Mr. Assad’s calculus, but stronger responses carry significant risk of escalation.
• Escalation could create several new problems, including the collapse of the Syrian government, which could prolong the war and sow chaos for millions of Syrians. It could also invite a direct military confrontation with Russia, which warned that it would shoot down any missiles.