Mr. Assad may think he can act with impunity now. After all, Russia, which intervened militarily in 2015 to save him from defeat by rebels, vetoed a United Nations Security Council resolution in February that would have punished Syria for using chlorine-filled barrel bombs in 2014 and 2015.

Now comes the Trump administration, which has made clear that ousting Mr. Assad is not a priority and fighting the Islamic State takes precedence. President Barack Obama, after calling for Mr. Assad’s ouster in 2011, shifted toward that same view, but only after repeated efforts to work with Russia on a political solution. Mr. Obama also had a record of condemning Mr. Assad’s atrocities and urging that he and his allies be prosecuted for war crimes.

On Tuesday, Mr. Trump called the attack a “reprehensible” act “that cannot be ignored by the civilized world.” The usually invisible secretary of state, Rex Tillerson, did better. He condemned Mr. Assad by name, said he must be held accountable and pointed out that Russia and Iran “also bear great moral responsibility for these deaths.”

But the comments have little power, coming as they do after weeks of Mr. Trump voicing both distaste for America’s traditional role as a promoter of human rights and praise of authoritarian leaders, like Vladimir Putin of Russia. Mr. Trump also blamed Mr. Obama for the new attacks, citing his “weakness and irresolution” in setting a red line in 2012 against chemical weapons and then doing “nothing.” Has he conveniently forgotten September 2013 tweets telling Mr. Obama “do not attack Syria”?

In his statement, Mr. Trump ignored the fact that instead of taking military action, which Congress mostly opposed, Mr. Obama worked with Russia on a deal under which Mr. Assad agreed to dismantle his chemical munitions. Although much of the stockpile was destroyed, international inspectors later found Syria retained some capability.