Hezbollah has won grudging respect, even from some foes, for its tenacious guerrilla campaigns against Israel. But now Lebanon’s most powerful military organization is losing its aura of invincibility.

Tactics that the Shiite group has used against Israeli soldiers are being inflicted on its own forces by militants from Islamic State and the al-Qaeda-affiliated Jabhat al-Nusra, who have carved out footholds in Lebanon along the porous Syria border.

The growing number of attacks and kidnappings by the Sunni militants represent the opening of yet another military front for Hezbollah. It already has thousands of troops deployed in Syria to bolster President Bashar al-Assad’s regime, while still more are facing Israel in southern Lebanon. […]