Two years into the fight against Isis, more than 15 of the group's most senior members have been killed by airstrikes. They include Baghdadi's former deputies Abu Muslim al-Turkmani and Abu Ali al-Anbari, Omar al-Shishani, the head of the group's chemical weapons programme in Iraq, Abu Malik and numerous regional leaders in Anbar and Nineveh province. Last year Baghdadi very nearly joined them.