The radical Shi'ite group is watching the unrest in neighboring Syria with alarm and is determined to prevent the West from exploiting popular protests to bring down Assad.
Hezbollah supported pro-democracy movements that toppled Western-backed leaders in Tunisia and Egypt, but officials say it will not stand idly by as international pressure mounts on Assad to yield to protesters.
It is committed to do whatever it takes politically to help deflect what it sees as a foreign campaign against Damascus, but it is also readying for a possible war with Israel if Assad is weakened.
"Hezbollah will never intervene in Syria. This is an internal issue for President Bashar to tackle. But when it sees the West gearing up to bring him down, it will not just watch," a Lebanese official close to the group's told Reuters. "This is a battle for existence for the group and it is time to return the favor (of Syria's support). It will do that by fending off some of the international pressure," he added.
Hezbollah believes the West is working to reshape the Middle East by replacing Assad with a ruler friendly to Israel and hostile to itself.
Analysts rule out the possibility of a full-scale regional war involving Syria, Iran and Lebanon on one side against Israel backed by the United States. A war pitting Hezbollah against Israel was more likely, they said.