This week’s Brussels attacks give a reminder that the IS threat goes well beyond its Syria heartland. In Libya, Europe has an IS mini-state on its doorstep, whilst the attack on Algeria’s In Salah gas plant shows even the region’s strongest state is vulnerable. Progress in Syria absent an overall strategy risks hastening the metastasis rather than the eradication of the group’s jihadistic nihilism.

This week’s attacks on Brussels have once more underlined the...