Allying themselves with the Shiite militant group Hezbollah, some Lebanese Christians are standing their ground against ISIS.

RAS BAALBEK, Lebanon—Rifaat Nasrallah sits impatiently on a sofa in his home in the small village of Ras Baalbek sipping coffee and chain smoking Marlboro Lights. A handgun casually stuck in the waistband of his jeans, he says he’s only in town briefly; these days he’s been spending more and more time in the hills just outside his living room window, where he commands an all-Christian militia that he credits for keeping Islamist militants in neighboring Syria at bay.