The scale and horror of Tuesday’s gas attack on civilians in Idlib highlighted the vacuum in the Trump administration’s foreign policymaking: the incident was met first by silence, then by criticism of Barack Obama.

Donald Trump described the attack, which killed scores of victims, including many children, as a direct “consequence” of his predecessor’s Syria policy.

“These heinous actions by the Bashar al-Assad regime are a consequence of the last administration’s weakness and irresolution,” he said in a statement. “President Obama said in 2012 that he would establish a ‘red line’ against the use of chemical weapons and then did nothing.”

The reflex illustrated Trump’s enduring sense of being in his predecessor’s shadow, reinforcing the impression given by his obsessive tweeting of unsubstantiated claims that Obama wiretapped him.

As with healthcare, Trump’s policy on Syria has been defined by the desire to unpick Obama’s legacy – without a clear picture of what would replace it.

In the absence of a clear vision, the initial response to on Tuesday was silence.

The Idlib attack was swiftly condemned by western capitals and congressional leaders, but the US secretary of state, Rex Tillerson – who was visiting neighbouring Jordan at the time – ignored a press question about it, retaining his customary silence in the face of daily world events.