The escalating role in the Syrian conflict, supporting longtime ally President Bashar Assad against rebels battling to topple his regime, is becoming a gamble for the Shiite movement that could weaken its campaign against Israel and its standing in Lebanon.
The rebel Free Syrian Army claims 7,000 Hezbollah fighters are deployed in Syria. There's no way to verify that, but a force that size would involve most of Hezbollah's military wing despite a major recruitment drive after the 2006 war with Israel.
That seems unlikely since it would presumably mean pulling combat forces out of the Israel front in south Lebanon.
French Foreign Minister Laurent Fabius says France's intelligence service estimates 3,000-4,000 Hezbollah fighters are in Syria.
Some 1,700 were involved in the battle for Qusair that began May 19, when regime forces launched a major assault on the rebel-held town in western Syria, Lebanese security forces said. Hezbollah has been tight-lipped about its casualties, which suggests they may have been much higher than anticipated.
Two senior field commanders were reported killed in Qusair. Lebanese sources identified them as the commander of the Al-Quds Brigade, known by his nom de guerre of Abu Ajib, and his deputy Hamza Ramloush.
One Hezbollah fighter, who was in action until May 25, told reporters in Beirut the movement had lost about 100 killed in Qusair. But he said "we'd planned for as many as 1,000 martyrs" because the rebels were well dug in on their own ground.
Hezbollah initially denied its military wing was fighting in Syria and sought to bury its dead unobtrusively. But it's been forced to admit it's heavily engaged.
Now the extent of its casualties are becoming known, some in the movement question why it should be fighting fellow Arabs in Syria when its main mission has always been battling Israel.
And while there have long been differences within Hezbollah between pro-Syria and pro-Iran factions, party leader Hassan Nasrallah says the movement will become totally isolated if Assad's toppled and its arms supply route from Iran is severed.
Even so, fighting in Syria is a big gamble for Nasrallah. If Hezbollah is isolated, it would be caught between rival Sunni militants, including al-Qaida, and Israeli forces, and many Lebanese who blame it for dragging the country into wars that were not of their making.
It is clear Hezbollah's commitment of a sizeable combat force was the decisive stroke that allowed Assad to effectively seize Qusair.
Hezbollah's fighters are highly disciplined and well-trained by Iran, particularly the elite al-Quds Force, the Revolutionary Guard Corps' largely covert foreign operations arm which is closely involved in bolstering Assad's forces.
Indeed, Hezbollah's configured more as a conventional military force than the sectarian militia that's long been its image. It's also had more experience in urban fighting that Assad's regulars, which is why, after a string of tactical advances, it was made the spearhead of the assault on Qusair.
Most of Hezbollah's casualties were inflicted in the opening stage of the offensive, Lebanese security forces said. At one point, a 200-man assault force was badly mauled in an elaborate bomb ambush by the rebels and lost at least 18 killed and dozens wounded.
Elite Hezbollah units were then sent in to rally the attackers and pushed deeper into Qusair despite fierce opposition.
The mounting losses may intensify pressure on Nasrallah to ease off, not just for political reasons but because the casualties will erode Hezbollah's military capabilities, particularly against Israel.
It's generally accepted that Hezbollah lost 500-600 fighters during the 34-day war with Israel in 2006.
That was deemed a significant percentage of its fighting force at the time, around 2,000 hard-core regulars.
It has greatly increased recruitment since then, maintaining a larger force.
"A high casualty rate of newly trained 'elite' fighters, recruited to replace those killed in 2006, means a waste of precious time and resources," observed analyst Tony Badran of the Foundation for Defense of Democracies.
"The loss of even more battle-hardened soldiers, on top of the 500-600 men from 2006, means further loss of operational memory and combat experience in the party's fighting corps."