How are President Obama and his state secretary, John Kerry, going to account for themselves in the latest gas attack by the Syrians on their own people? It was fewer than four years ago that the duo shrank from enforcing a red line the president himself declared against gas attacks in Syria. They assured us that the right approach was the gas of diplomacy. Now the toll is scores of Syrians lying dead or gasping for life on the soil of Idlib Province.

Videos of the tragedy, which are now starting to go up on the World Wide Web, are almost unbearable to watch. It may be that the overall toll — at least so far — is below that of the attacks President al-Assad launched in 2013 at Ghouta, where more than 300 were slain (or that Saddam Hussein launched in 1988 at Halabja, where as many as 5,000 were slain). The attack at Idlib is nonetheless not only a tragedy but a moment for accounting.

We do not intend to suggest that blame attaches directly to Messrs. Obama and Kerry for the war crime that has just been committed. That culpability lies only with the perpetrators. The crime does, though, shed a sickening light on the video of Mr. Kerry and his Russ counterpart, Sergey Lavrov, boasting in 2014 about the “framework” they’d negotiated and declaring themselves “committed,” as Mr. Kerry put it, “to the elimination of Syrian chemical weapons.”

No doubt it would be too much to hope that this might enable the Democrats to discover a gluon of humility in respect of President Trump. His administration has been maneuvering to put a priority on the defeat if the Islamic State at the expense, at least for the moment, of the liberation of Syria from the regime that is using poison gas. The defeated Democrats are seeking to tar Mr. Trump for his preparedness to work with the very Russia with whom the Democrats dickered.

The thing to remember here is that the key to Syria is not Russia but Iran, which remains the top enemy in the Middle East. There is every reason to expect that regime change in Iran would produce an earthquake in Syria that would end the Assad era. Regime change in Iran, however, is going to be much harder because the Obama-Kerry combination shrank from confronting the mullahs, too, and placed them on a course to obtain an a-bomb. The gas of diplomacy indeed.