At the entrance to this village in Hezbollah’s Bekaa Valley heartland, under a sign welcoming visitors to “The Citadel of Resistance,” workers on Monday hoisted a freshly printed banner honoring a young man described as one of Hezbollah’s latest martyrs — killed in battle not with Israel, the foe the group’s guerrillas train to fight, but with Syrian rebels. Down the road, another dead fighter’s uncle, Fayez Shukor, welcomed mourners under a tent overlooking the valley as the sun set on a day that had seen Hezbollah’s death toll rise to unexpected heights as the group joined Syrian forces trying...