Meanwhile, Hezbollah’s victories have reassured Russia and Iran that Assad’s troops will continue to be augmented by an arguably more effective and motivated fighting force. Weeks before the Qusayr campaign, in April, Mr. Nasrallah met with Iran’s Ayatollah Khamenei in Tehran and with Russia’s deputy foreign minister in Beirut, presumably to coordinate their positions. Since the Qusayr offensive, Moscow has remained conveniently silent, and Tehran has boosted its troop presence in Syria, according to ground reports.

Israel more than most stands to lose from a resurgent Hezbollah. Despite sustaining hundreds of losses in Qusayr alone, Hezbollah’s fighters are gaining valuable combat experience that could be useful in a future conflict with Israeli forces. After witnessing Hezbollah’s ability to capture large swaths of territory in Syria, Jerusalem can no longer shrug off Mr. Nasrallah’s threats to invade Israel’s Galilee region in the next war.

Iran is also likely to continue leveraging the Assad regime to transport weapons to Hezbollah’s coffers, despite threats of additional Israeli airstrikes. Hezbollah’s acquisition of Iranian anti-air, anti-ship and surface-to-surface missiles would not only guarantee Mr. Nasrallah long-term military hegemony within and without Lebanon. It would also provide Tehran with a greater deterrent against any future Israeli strike on its nuclear facilities.