Amid the tangled streets of Beirut’s southern reaches, mournful Hezbollah followers have turned the mausoleum of Hassan Nasrallah, the group’s leader for more than 30 years, into a pilgrimage site.

His grave, a white marble slab engraved with Quranic verses, is sheltered by a canopy adorned with white and yellow artificial flowers. Around it, men and women pray separately on a patchwork of carpets.

Mr. Nasrallah was assassinated in the fallout from the Oct. 7, 2023, Hamas attack on Israel, which set off the war in Gaza. The Lebanese militant group Hezbollah opened a second front in solidarity with Hamas by firing across the Lebanon-Israel border. The conflict eventually escalated into a full blown war, during which Israel eliminated Hezbollah’s leadership.

The Iran-backed group’s power has ebbed ever since, and after decades of building a formidable state within a state, Hezbollah faces an uncertain future.

“Hezbollah was two things: It was Hassan Nasrallah and the weapons,” said Mustafa Fahs, a Lebanese political commentator.

Now, at one of its weakest moments, Hezbollah finds itself struggling to hold on to the weapons that are at the very core of the group’s identity.