Under pressure from Iran and their own self interest in seeing Assad survive, Hezbollah is committing its men, after Qusayr, to fight the Syrian rebels:
Sheikh Hassan Nasrallah, chief of the Shiite Hezbollah group in Lebanon, appeared unwavering in his support for the regime of Syrian President Bashar Assad.
He signaled for the first time the Iranian-backed militant group will stay involved in the civil war after helping Assad's army recapture the key town of Qusair in central Homs province from rebels.
"We will be where we should be. We will continue to bear the responsibility we took upon ourselves," Nasrallah said in a speech via satellite to supporters in south Beirut.
From what I've read, we're talking several thousand Hezbollah light infantry. And they endured hundred of casualties (dead and wounded) at Qusayr. Without enough trained Syrian infantry and because the new Assad militias aren't well trained, the Hezbollah lads get to be the shock troops of the Assad counter-offensive. How many times can Hezbollah bleed at that level? Religious fanaticism is one thing. But the organization as a whole isn't suicidal.
Despite their loud bluster of commitment, I assume Hezbollah has established some minimum force they deem necessary for home defense and won't cut into that to send to Syria, no matter how much pressure they face from Iran. After all, Hezbollah would hardly consider it worthwhile if they preserve their rear area by successfully defending Assad, but then lose their base if the Israelis pounce on a depleted Hezbollah in Lebanon secure in the knowledge that Assad's forces are too beat up to interfere.
If we do this right, Iran, Assad, and Hezbollah all lose. Russia sustains a setback, too, but honestly I'm not too worked up over them, one way or the other.
UPDATE: The Israelis say Hezbollah has already suffered over a thousand casualties. That's a third to a fifth of what they sent. Some are lightly wounded, of course, and could return to the fight. But still, that's a lot. Hezbollah can't be the shock troops for long with that kind of price.