It is no coincidence that Saturday’s deadly chemical attack on the suburb of Douma, which killed 49 people in the last rebel-held area, came a year after Mr. Trump’s airstrikes on Assad targets. Is this Mr. Assad — or “Animal Assad,” as Mr. Trump referred to him in a weekend tweet — sending a deadly message to Washington following the decision to pull out troops? Is it a way of Mr. Assad telling the international community that he can do whatever he wants, knowing he will never be punished?

Realistically, the 2,000 American troops have not made a huge difference to the landscape of the war in terms of humanitarian assistance, because the United States never had a vested interest in protecting the Syrian population; the troops were not deployed in a way that, say, could ensure the delivery of food or medicine, or open up besieged towns. But the signal their sudden withdrawal sends to the Syrian people, especially the Syrian Kurds, and the rest of the world will be damning.

Kassem told me that living under the Assad dictatorship for 40 years, the national ideology taught him and his friends that America was the devil. “We were taught that America was the enemy,” he said. “Then we figured out it was all propaganda. But after seven years of atrocities, do you know what my friends and people around the Middle East are saying? That America is the enemy again. Because they see the Russians bombing us and the United States doing nothing. Now they pull out — when they could have been our friend or ally.”

Of course, Mr. Trump is not concerned about these things; his sole metric for success is defeating the Islamic State — though the vast majority of Syrian civilians were killed by Mr. Assad’s forces. But any claim of victory over the Islamic State is premature and naïve. As Kassem and others note, the seeds of “ISIS 2.0” are already planted in the thousands of angry people who have lost their families, their homes, their country. “When there are a million people dead,” Kassem said, “when most have lost everything, ISIS will say, ‘We told you so.’”

Mr. Trump has said that with America gone, its regional allies, especially Israel and Saudi Arabia, should pick up the slack. But that could make things even worse. Israel has a long and complicated relationship with Syria, and it has shown little willingness to get more involved. And Mr. Trump is perhaps forgetting that Saudi Arabia is the birthplace of the hard-core Wahhabi branch of Islam, which has inspired jihadists around the globe and could turbocharge a revived Islamic State.

As long as America has had troops in Syria, there was at least hope for a peaceful resolution to the war. Now the bottom is falling out. Kassem says he hears talk about the coming of the Mahdi, whom many Muslims believe will bring about Judgment Day, because the region is engulfed in chaos — a precondition for his arrival. “Everyone is talking end of days,” he says.