The deal to let IS fighters escape from Raqqa – de facto capital of their self-declared caliphate – had been arranged by local officials. It came after four months of fighting that left the city obliterated and almost devoid of people. It would spare lives and bring fighting to an end. The lives of the Arab, Kurdish and other fighters opposing IS would be spared.

But it also enabled many hundreds of IS fighters to escape from the city. At the time, neither the US and British-led coalition, nor the SDF, which it backs, wanted to admit their part.

Has the pact, which stood as Raqqa’s dirty secret, unleashed a threat to the outside world – one that has enabled militants to spread far and wide across Syria and beyond?