I know, Muslims shelling other Muslims, who cares anymore? But here's the quick rundown: it all started in 2005 with the killing of former Lebanese Prime Minister Rafik al-Hariri. Shortly afterwards, the UN Security Council passed Resolution 1636, to set up a Special Tribunal to try Syrian suspects in the assassination. In order to prevent Lebanon from getting this tribunal up and running, Syria has instructed Fatah al-Islam to take actions to destabilize the Lebanese government. Fatah al-Islam, created by Syria, was formed by splitting off Fatah al-Intifada after Syria pulled its army out of Lebanon in April 2005.

Lebanese forces are prevented from physically entering any of the Palestinian camps because of a 1969 Arab accord. There are over 400,000 Palestinians spread out over 12 camps throughout the Lebanon.

Related:

Smooth Stone,
Dozens Slain as Lebanese Army Fights Islamists

Fierce clashes erupted between Lebanese Army soldiers and the Islamic militant group Fatah al-Islam in the vicinity of a Palestinian refugee camp in Tripoli on Sunday, leaving 22 Lebanese soldiers and 17 militants dead and dozens injured. The confrontation raised fears of a wider battle to rout militants in the rest of Lebanon's 12 refugee camps, where radical Islam has been gaining in recent years. The ranks of religious militants bent on a broader jihad have swelled, as some have traveled to Iraq to join the insurgency there and, more recently, have returned to establish movements of their own within the camps. Fatah al-Islam has been a growing concern for security authorities in Lebanon and much of the region.

Gateway Pundit,
Death Toll at 65 in Lebanon- Wanted Terrorist Among DeadDeath Toll at 65 in Lebanon- Wanted Terrorist Among Dead

One of the men killed in Sunday's fighting, Saddam El-Hajdib, was a suspect in a failed German train bombing, a sign that Nahr al-Bared refugee camp had become a refuge for militants planning attacks outside of Lebanon. In fact it had been reported that the Al Qaeda-linked group was in the advanced planning stages for spectacular external attacks against civilian targets in Europe and the America.

From Beirut to the Beltway,
Explosion in Verdun

Breaking News: An explosion rocked the upscale area of Verdun in Beirut. LBC is airing footage showing extensive damage to residential buildings and restaurants near the Dunes center, and not far from parliament speaker Nabih Berri's residence.
[...]
And there you have it. The Assad regime is waging a war on Lebanon by ripping it apart.

Now the Lebanese are fighting back thanks to US Aid.

Good Neighbors Blog,
21 May 2007,
March 14 Strikes Back…Finally

In 2005, in the midst of assassinations and random bombings in Christian civilian areas, US Secretary of State Condoleeza Rice visited Prime Minister Siniora and offered the assistance of the United States to strengthen Lebanese government institutions. The goal of both Rice and Siniora was to strengthen Lebanon’s security apparatuses, and the US donated millions of dollars to help the police investigate the assassinations.

The police now have a much wider presence in Lebanon. New vehicles painted with distinctive colors and with brightly flashing lights now patrol the streets of Beirut. Allegedly, the police have also infiltrated some terrorist groups, and use the intelligence they gather to take action before violence occurs. After Siniora sent his letter to the United Nations calling for an international tribunal to try Shaheed Rafiq al Hariri’s assassins, all cars on both Bliss and Hamra Streets were removed.

The attack on Nahr al Bared is an example of 14 March leaders being proactive. Instead of waiting for the next round of assassinations and for more civilian buses to be blown up by militants operating out of Nahr al Bared, the government is taking any opportunity they get to strike back.