A world grown numb to the slaughter of civilians in Syria has been roused in the last 48 hours by photographs on social media of lifeless men, women and children in the rebel-held town of Douma, many with white foam coming from their mouths and nostrils, victims of chemical weapons. Outraged Western nations blame President Bashar al-Assad’s regime and demand retaliation.

Russia and Iran, Mr. Assad’s callous enablers, have denied that he has once again used these horrific weapons on his own people. But Douma is surrounded by Syrian forces, whom experts have blamed for most of the 85 chemical attacks in the country over the past five years. Syria had a major chemical weapons program before pledging to surrender it after chemical attacks in 2013, a commitment it failed to fully honor.

President Trump took limited military action against Syria after a chemical weapons attack last year, largely ignored the issue after that and then last week surprised his military commanders by announcing plans to soon withdraw 2,000 troops in the fight against the Islamic State. Mr. Trump vowed on Sunday on Twitter that there would be a “big price to pay” for the latest killings, estimated at up to 70 people dead, according to aid groups.

But the president should know by now that tough talk without a coherent strategy or follow-through is dangerous.