Early in his presidency, Donald Trump knew exactly whom to blame for the chemical weapons used in Syria, and what to do about it. The “heinous” sarin gas attacks by President Bashar al-Assad on the Syrian town of Khan Sheikhoun last April happened because President Barack Obama “did nothing” to enforce his red line against the banned arms after an attack near Damascus in August 2013, Mr. Trump said.

So Mr. Trump ordered the launch of 59 cruise missiles against a Syrian airfield where the April chemical attack originated. Invoking the horror of “innocent babies” choked by poison gas, he said military action would “deter the spread and use of deadly chemical weapons.”

Only it didn’t.

In the 11 months since then, there have been many such attacks, including at least six this year, which American officials and human rights groups blamed on Mr. Assad. Rather than the more lethal sarin agent used in April, recent attacks reportedly have involved chlorine.

The use of poison gas, a war crime under international law, has been integral to Mr. Assad’s scorched-earth drive to regain control of the last rebel-held areas near Damascus. By bombing civilians and depriving them of food and medical care, he has killed more than 700 people in the past several weeks, on top of nearly 500,000 killed since the civil war began in 2011.