The problem is, Hezbollah is not a big force and seeing how Israel deal with them as well as southern Lebanon in 2006 would mostly make them think twice before any action. And that is before the US offering Israel a no-bar support like this.
And that is just one part of the issue, another one is that it seems like Hamas did not really communicate well what they are doing to both Iran and Hezbollah, or they did tell Iran (or they even plan it, who knows) but did not tell Hezbollah, but that's strange considering Hezbollah is all but an Iran satellite. Which mean Hezbollah was not prepared for anything to exploit this. How do I know? Usually when one group do this attack, if other group know the plans ahead their support message comes almost immediately, Iran releases their message almost the first time the news of these attack broke, while Hezbollah's came around mid-day, which indicate they were not aware of and trying to confirm such attack did happen.
Had they been prepared, they would have made the move almost immediately with Hamas, but that is not to be, and 6 hours after the initial attacks, the Israeli already fortified their border, any Hezbollah move would be pointless now.
