By Roger Hardy
Middle East analyst, BBC News
Lebanese forces are now fighting on a new front - in the south
For more than two weeks, the Lebanese army has been laying siege to militants holed up in the Palestinian refugee camp of Nahr al-Bared in northern Lebanon.
Now it is facing a similar enemy on a second front with the outbreak of fighting around the Ain al-Hilweh camp near Sidon in the south.
In each case, Lebanese forces find themselves confronting jihadi groups of the al-Qaeda type: Fatah al-Islam in the north and Jund al-Sham in the south.
Both are tiny offshoots of bigger movements.
In the case of Ain al-Hilweh - the largest of the dozen Palestinian refugee camps in Lebanon - the main radical Islamist group is Usbat al-Ansar (the League of Partisans), whose roots go back to the mid-1980s.
The story of how the jihadi groups secured a foothold in the camps is told by French expert Bernard Rougier in his newly translated book Everyday Jihad: The Rise of Militant Islam among Palestinians in Lebanon.
According to Mr Rougier, Usbat al-Ansar was initially led by a charismatic Palestinian, Hisham Sharaydi.
Under Sharaydi, the group co-operated with the Iranian-backed Lebanese Shia movement Hezbollah.
In the climate created by Israel's invasion of Lebanon in 1982, he believed Sunni and Shia should work together against their common enemy Israel.
But in 1991 Sharaydi was assassinated and his successors broke the Iranian connection to follow an explicitly Sunni form of militancy.
Usbat al-Ansar is hostile both to the Arab regimes and to the PLO, the main Palestinian umbrella organisation which the Islamists view as excessively secularist.
In 2002, Jund al-Sham - literally the Army of Greater Syria - broke away to form an even more radical splinter group.
Islamists do not accept the borders the region inherited from European colonialism. For them, Syria, Lebanon and Palestine form one Muslim land.
But despite its name, there is no clear evidence the group has ties to the Syrian government, as some Lebanese allege.
Ain al-Hilweh is the largest Palestinian camp in Lebanon
It is true that Syria benefits from the current situation, which is highly destabilising for Lebanon and its current anti-Syrian government.
But even though the Syrians have sought to meddle in the camps, it is probably an over-simplification to regard the radical groups as mere instruments of Syrian policy.
It seems likely that Jund al-Sham, like Fatah al-Islam, is not much interested in the internal Lebanese political game.
With their links to Iraq and radical Islamist networks elsewhere in the region, these groups see themselves as part of the much bigger battle between Islam and the West.
As such, they pose a tough new challenge not only to Lebanon but to the region as a whole.