At one point, Barack Obama called the use of chemical weapons a “red line” that would prompt US intervention in the Syrian civil war. The red line reportedly got crossed again today in what observers insist was a sarin gas attack on Idlib, which killed dozens of people, including eleven children at the latest coun:

Suspected chemical weapons attack in Syria could be among deadliest in civil war; activists say at least 58 killed https://t.co/syY4EY1PHd pic.twitter.com/6fr19M7mG8

— CBS News (@CBSNews) April 4, 2017

Airstrikes carried out by Syrian government or Russian jets hammered rebel-held areas of the northwestern Idlib province on Tuesday leaving dozens of civilians dead, according to activists and medical workers who say chemicals weapons were likely used.

Horrific video and photos posted online by the Syrian Civil Defense, the volunteer first-responders organization commonly known as the White Helmets, showed young children who had purportedly died in the attack on the town of Khan Sheikhoun.

A member of the White Helmets team in Idlib posted a message on Twitter saying at least 37 civilians were killed and hundreds injured in what he called a “poisonous gas attack” on the town. The images he posted showed children — at least eight in one video — seemingly deceased without any apparent external injuries.

The Syrian Observatory for Human Rights (SOHR), a U.K.-based activist group that relies on a network of contacts inside Syria and which generally proves an accurate source of information, said at least 58 people were killed, including 11 children. SOHR said the strikes left many victims chocking for air, and cited local medical workers as describing the effects of a poison gas attack.

The only entities flying missions over Idlib are the Syrian and Russian militaries. Both denied dropping any chemical weapons at all, on Idlib or anywhere else, but Turkish president Recep Tayyip Erdogan got on the phone to Vladimir Putin to protest anyway:

Turkish President Tayyip Erdogan and Russian President Vladimir Putin discussed a suspected gas attack on Syria’s rebel-held Idlib on Tuesday, Turkish presidential sources said. …

“President Erdogan touched on the use of chemical weapons against civilians in Idlib. President Erdogan said such inhumane attacks are unacceptable,” a statement attributed to presidential sources said.

Not long after the gas attack, another sortie by military aircraft targeted a field hospital that had been treating some of the victims, according to opposition forces:

Syrian opposition activists say an airstrike has hit a small field hospital in a town in northern Syria where a suspected chemical weapons attack took place earlier in the day.

The head of the opposition’s civil defense force in Khan Sheikhoun, in Idlib provice, says the hospital was struck hours after the alleged gas attack that killed dozens of people.

The man who goes by the name of Abu Hamdu says the medical point has been leveled and five rescue vehicles were damaged. It wasn’t clear if anyone was killed.

He says warplanes “targeted us after the attack.”

If the use of sarin gas gets confirmed by neutral observers in Syria, that puts the onus for a response from the United Nations and the West. Sarin gas is prohibited for use by militaries, especially against civilian centers. France has now called for an emergency UN Security Council meeting to deal with the violation:

“A new and particularly serious chemical attack took place this morning in Idlib province. The first information suggests a large number of victims, including children. I condemn this disgusting act,” Jean-Marc Ayrault said in a statement.

“In the face of such serious actions that threaten international security, I ask for everyone not to shirk their responsibilities. With this in mind, I ask for an emergency meeting of the U.N. Security Council,” he added.

What will the US do in response? Obama’s credibility on the international stage never recovered from his red-line retreat. So far Donald Trump has not offered any such warning, but he has signaled a willingness to let Bashar al-Assad stay in power even after the earlier use of chemical weapons. John McCain angrily blamed Trump for this new escalation because of those signals:

Sen. John McCain is blaming the Trump administration for a chemical weapons attack in Syria Tuesday morning that killed dozens of people. ..

The attack comes days after the Trump administration went public with its policy that it would allow Syrian President Bashar Assad to stay in office in order to find an end to the conflict. McCain said those comments empowered the Syrians and their allies, Iran and Russia, to possibly drop chemical weapons on their people.

“We’ve seen this movie before when Barack Obama said we would have a red line, they crossed it and we did nothing,” McCain said, “Bashar al-Assad and his friends, the Russians, take note of what the Americans say … and I’m sure they took note of what the administration said.”

“I’m sure they’re encouraged and know the United States is withdrawing.”

Certainly the eyes of the world will be on Trump now after this attack. Will he get tough with Assad and Russia over this attack, or will he continue to take a more laissez-faire approach to Syria? The latter will almost certainly send more signals to Assad that the US has little interest in how he treats his people, but the latter could land the US in a hot war with not just Assad but potentially also Russia. Had Obama intervened in 2013 — and more importantly, had he worked with Congress to prepare the political ground here in the US to do so — Russia might have remained on the sidelines. The only path open to Trump now is to talk tougher and push hard for a UN crackdown, forcing Russia to veto it and isolating them a little more in the diplomatic realm.