Note that Dr Tennent says that "We, of course, encourage governments around the world to act. Violence even in the name of Allah must be stopped. ISIS must be defeated. Relief must be given. Justice must prevail. Christians, Muslims, Yazidis and other innocent people in the region must be protected."
But the crux for him is simply that "the long-term solution is not military action, but the gospel of Jesus Christ." The same gospel which calls us to "love our enemies and to pray for those who persecute us."