The attack raises fears on both sides of a repeat of the 2006 Lebanon war
It is not that long ago that Israel was waging war in Gaza when Hezbollah militants opened up a second front with Lebanon.
In July 2006, Israeli troops were two weeks into their unsuccessful campaign to rescue captured soldier Gilad Shalit when the powerful Shia Muslim political and militant movement attacked from the north.
Its fighters launched dozens of Katyusha rockets and mortars at Israel and seized two more soldiers and killed eight others in cross-border raids.
It was seen as a dramatic gesture of solidarity with the Palestinians, but Israel's response was far more dramatic and devastating to Lebanon.
Hezbollah said it wanted to exchange the two soldiers for thousands of Palestinians held in Israeli detention.
What it got was a 34-day onslaught from the Israeli military, costing more than 1,000 lives, mostly Lebanese civilians.
About 160 Israelis were killed, mostly soldiers, in fighting and rocket fire from Hezbollah. Hundreds of thousands of civilians were displaced on either side.
Events in Lebanon during the summer of 2006 ended up completely overshadowing what had been going on in Gaza.
We are now nearly two weeks into Israel's campaign to hit the Hamas militant movement in Gaza, an attempt to reduce rocket fire by Palestinian militants into Israeli territory.
The news of rockets being fired from Lebanon will have raised fears of a possible serious escalation.
But this new year period has already been so bloody in Gaza - with 700 Palestinians killed, along with 11 Israelis - that it looks as though Lebanon will be the sideshow this time.
Intense speculation followed Thursday's rocket fire as to whether Hezbollah was behind it, or one of the other armed militant groups operating in Lebanon.
There was no immediate public denial, although analysts concluded Hezbollah were unlikely culprits, despite recent fiery rhetoric from the group's leader Hassan Nasrallah about the possibility of renewed conflict with Israel.
But reports from Beirut said Hezbollah had made it known to the Lebanese government that it was not involved.
And the group has a reputation, even among Israelis, for being a credible conveyor of information about its activities.
Also the scale of the attack - much smaller in the first instance than the July 2006 ambushes - argued against Hezbollah involvement.
The group would be capable of a much heavier blow if it had wanted a confrontation with the Israelis.
The rockets seem to have been short-range, and were fired from south of the Litani river, which is controlled by the Unifil peacekeeping force and the Lebanese regular army.
Southern Lebanon is a very difficult area to secure by regular troops
Hezbollah has largely respected the ban on armed activity by militants between the Litani and the Israeli border which has been in force since the August 2006 ceasefire, though the movement still has a strong presence in the area.
And it is doubtful that Hezbollah would attempt such a blatant challenge to UN authority at this time. Since May 2008 it has been part of the Lebanese government - with a power of veto on legislation - and would be unlikely to want to jeopardise that position.
That does not mean Hezbollah did not in some way have prior knowledge or consent to this attack. By reputation, not a leaf can move in southern Lebanon without its people knowing about it.
If so, Hezbollah may be wanting to show solidarity with Gaza without provoking a massive Israeli retaliation.
Last year, Israeli officials threatened that any attack from Hezbollah would trigger a retaliation against all of Lebanon that would make 2006 seem mild.
So who would attack Israel like this? Lebanon plays host to 400,000 Palestinian refugees, a reservoir of anger and militancy fuelled by 60 years of exile from what they consider as their land.
There are large refugee camps around Tyre and Sidon in southern Lebanon and militant groups have been known to launch rockets at Israel.
The last occasion of rocket fire was in January 2008, which was linked to the visit of US President George W Bush to Israel.
Hezbollah denied responsibility for that attack and the Israeli military blamed an unnamed Palestinian organisation.
But it need not be Palestinians - any small extremist group would find it relatively easy to smuggle some missiles into southern Lebanon and fire them off at Israel.
This is because the Unifil/Lebanese army regime is not in total control south of the Litani. It is notoriously difficult terrain to secure completely, as the Israeli army found to its cost during its long occupation of the area.