Idlib began trending on Twitter this morning as news broke that Bashar al-Assad’s regime killed dozens of civilians — many of them children — using chemical weapons in rebel-held Khan Sheikoun. This attack comes less than a week after U.S. Secretary of State Rex Tillerson announced that “the Syrian people will choose Assad’s fate,” effectively ending the Obama administration’s “Assad must go” policy.

Today’s massacre is a byproduct of this shift in U.S. policy even though the change is largely rhetorical rather than substantive. The more secure Assad feels, the more heinous his crimes against his own people will become.

The timing of today’s massacre is the result of cold political calculation, coming within President Trump’s first 100 days. Assad is testing Trump to see what his administration will and will not tolerate (to use an expression of Secretary of Defense James Mattis). When Assad’s sponsor Iran tested Trump by test-firing ballistic missiles, the Trump administration put Iran “on notice” and unilaterally imposed sanctions on certain individuals within the Iranian regime. There have been no further missile tests since then because the mullahs got the message.

The Trump administration’s reaction (or more likely, non-reaction) to today’s gassacre will shape how the regime wages war on the rebels and their civilian supporters for years to come. Exterminating dozens of civilians at a time using poison gas could become the new normal if Trump fails to adopt and enforce Obama’s “red line.” If that happens, we will see many, many more videos in the years to come like the ones below from today in Khan Sheikoun.