Lebanon’s Hezbollah is on the verge of launching an offensive against Syrian rebel groups in the mountainous Qalamoun region straddling the Syria-Lebanon border, north of Damascus. But what was once seen as a relatively minor engagement in the broader Syrian conflict has taken on new significance. After military setbacks in the north and south of the country in the past six weeks, the regime of President Bashar al-Assad is desperate for a victory and is depending on his Shia Hezbollah allies to deliver it. Assad and his Iranian and Hezbollah allies badly need to defeat — and to be seen...