In a ground mission in eastern Syria last weekend, US special forces killed Abu Sayyaf, a man they described as playing a key role in Islamic State's oil and gas operations. The American commandos were quickly engaged in a firefight, during which Abu Sayyaf was killed. But their original goal was to capture and interrogate him, apparently in an effort to improve their understanding of how IS works. It raised the question of how much is known about the structure of an organisation that rapidly overran large parts of Syria and Iraq last year, and has been able to hold...
