Israel, assuming this is a major unit air-dropped in. Syria's not going to go anywhere as long as the threat isn't direct. If it's a sizable Israeli force, they're not going to go toward Beirut, because that's been done before without success, so the only thing that would make sense would be to cut-off the Hezzie escape routes and scoop 'em up. Again, though, that'd be a sizable force, from the reporting so far it's not, so I'm still on the 'raid' page.
Brent Sadler from CNN was opining they went on a snatch and grab of Hezbollah leaders...