JUDY WOODRUFF: Our Margaret Warner is in the region and just visited a Lebanese city where dozens have already been killed in the sectarian violence fanned by the conflict in Syria.
And, as she reports tonight, the prospects for even further escalation there are high.
MARGARET WARNER: From the bluffs above Tripoli, Lebanon's second largest city, it looks like the thriving Mediterranean port it's been since antiquity. And not far from the seaside, the commercial district hums.
It's hard to believe that less than a mile from here, at the north end of town, a primitive sectarian war is raging between two poor neighborhoods, one Sunni, one of the splinter Shia sect the Alawites, who have lined up on opposing sides in the Syrian civil war next door.
In the past five weeks alone, three dozen people have been killed and at least 250 wounded in clashes that have deeply disturbed the rest of the country. It's the worst sectarian fighting Lebanon has seen in the nearly quarter-century since its own civil war ended in 1990.
For a closer look, we ventured first to the Sunni neighborhood Bab al-Tabbaneh, on the flat land within eyesight and rifle-shot of Alawite fighters in the Shia district of Jabal Mohsen above. The buildings were riddled by bullets and mortars. Yet, children were playing. And with rumors of a battle to come the next day, Lebanese army troops were taking up stations.
A neighborhood resident, Ahmed Jamal, gave us a tour of what he said once was called Gold Street.
AHMED JAMAL, Lebanon: Look at all this damage. All the shops have shut down and gone away.
MARGARET WARNER: Even worse than the physical damage, he said, is the psychological toll.
AHMED JAMAL: We live day by day. We don't know if we're going to be alive the next day. So, before we buy a house, we buy a cemetery plot or a coffin.
MARGARET WARNER: Jamal, who is a hummus maker, not a combatant, has bought guns to protect his family. He insisted the outbursts of fighting are always instigated by the Alawite district up the hill, whenever Assad's forces in Syria want to stir up trouble for Lebanon's Sunnis.
AHMED JAMAL: The orders come from higher command in Syria. Whenever they're under pressure, they give orders and the fighting starts here.
MARGARET WARNER: But he admits Sunni residents aren't blameless, carrying grudges against Alawites from more than two decades of Syrian occupation that ended eight years ago.
AHMED JAMAL: Their families were slaughtered, so when the children grew up, they bought guns and wanted revenge for their families.
MARGARET WARNER: The next block over, in streets of grinding poverty, preparations for battle were under way. From everyone, we heard a deep sense of grievance against Alawites.
From 21-year-old Ziad Habshiti a desire for revenge.
ZIAD HABSHITI, Lebanon: I'm fighting because they killed my brother. My brother was 13 years old, and they killed him.
MARGARET WARNER: What do you think you are accomplishing in this thing?
ZIAD HABSHITI: We are defending our rights, our land and our people.
MARGARET WARNER: Do you worry that you're destroying your neighborhoods?
ZIAD HABSHITI: Homes and houses are not important. People are dying. They are more important. The Prophet Mohammed said, whoever is aggressive against you, you should be against them just as well.
MARGARET WARNER: Just up the hill, a self-fulfilling prophecy, in the Alawite district of Jabal Mohsen. As his men fortified their fighting positions on the high ground, precinct commander Abu Ali Zumar boasted about why the casualty figures are lopsided in the Alawites' favor.
ABU ALI ZUMAR, Lebanon: We have machine guns, but we use the technique of sniping. We are organized people. We're not like them, who just take pills and run in the streets.
MARGARET WARNER: Like his Sunni antagonists, Zumar blames the rival side for instigating the clashes. But he nurses an even older sense of grievance, dating back to the split between Sunnis and Shia 14 centuries ago.
ABU ALI ZUMAR: It started with the killing of Hussain, and it will not end until judgment day.
MARGARET WARNER: He was referring to the slaying in 680 of the Prophet Mohammed's grandson, revered Shia saint Imam Hussain.
Less than five minutes away from the barricades, cafe manager Rabia Suleiman had invited us to sit with him on the street, packed with idle young men and dotted with posters of their fellow Alawite in Syria Bashar Assad.
RABIA SULEIMAN, Lebanon: There's a truce, but did you hear the gunshots?
MARGARET WARNER: I did. But who -- isn't that coming from here?
RABIA SULEIMAN: It's coming from their area.
MARGARET WARNER: Right now, we heard gunshots, but nobody is moving.
RABIA SULEIMAN: It is safe. The danger is on the demarcation line.
MARGARET WARNER: Despite his blase tone, Suleiman was ready for combat.
RABIA SULEIMAN: I'm a fighter. We are all fighters. Whenever it starts, we all become fighters. We carry weapons to defend our area. And we are expecting these events to spread throughout Lebanon.
MARGARET WARNER: Yet not far from this "Lord of the Flies" scene, we found humanity at a small eatery near the port in customer Ahmad Moustafa Mohammad.
AHMAD MOUSTAFA MOHAMMAD, Lebanon: We give them all the support they need, mattresses, blankets, food portions, and we give each family $100 dollars to help in the rent.
MARGARET WARNER: He heads a local aid organization helping some of the 3.25 million or more refugees who fled the real war in Syria, many to Tripoli and North Lebanon. Help is especially critical in the first three months, he said, before they get their bearings.
AHMAD MOUSTAFA MOHAMMAD: During these three months is when we step in and we help them. Otherwise, they would be sleeping in the streets and have nothing.
MARGARET WARNER: Despite funding from U.N. relief agencies and individuals and foundations mostly from the Gulf, he said, his group is struggling as the wave of refugees swells.
Mohammad took us to a 24/7 medical clinic his charity runs for refugees who are injured or ill.
ABDUL KARIM AL JERBIL, Refugee: They brought us through the mountains. They had to carry us on animals for transportation until we got here.
MARGARET WARNER: Twenty-four-year-old Abdul Karim Al Jerbil, a fighter with the Free Syrian Army, or FSA, was hit by an explosion in the early days of the battle for the strategic Syrian town of Qusayr, 40 miles away.
When we saw him, he was having a skin graft for his shattered forearm, where the skin and tissue had been blown off and left to fester for weeks.
DR. GHAZI ASWAD, Orthopedic Specialist: The situation here is very tragic.
MARGARET WARNER: The supervising surgeon, Dr. Ghazi Aswad, a French orthopedic specialist of Syrian descent who came here a year ago.
GHAZI ASWAD: The pain of the women, the pain of the children, the pain of the people made me come here.
MARGARET WARNER: But also suffering, as this city is sucked deeper into the Syria conflict, are ordinary Tripoli citizens, the men and women who make this city tick.
Normally, at midday, this tailor's souk in Tripoli would be bustling with shoppers. But with sectarian fighting and killings in the neighborhoods right next door, nobody knows when the crowds will return.
Forty-two-year-old Hassan Hamwie has seen his clothes-making and repair business drop nearly 80 percent since the Syria conflict began and Tripoli's rival neighborhoods revived their longstanding feud.
HASSAN HAMWIE, Tailor: My five-year-old asks me what's happening, and I'm telling him there are clashes. He asks me why there are clashes, and I have no answer.
MARGARET WARNER: He blames self-serving politicians, an ineffective army and divided Lebanese government and Syrian provocateurs on both sides for using Lebanon as a pawn.
So does it make you angry?
HASSAN HAMWIE: One hundred percent it makes me angry, because it has the worst impact on the middle and lower classes in Tripoli and in Lebanon.
MARGARET WARNER: Just down the row of shops are the shuttered doors of another tailor felled by a sniper in the fighting.
ADNAN AL KATAMI, Tailor: With all the civil wars and troubles we have had, there has never been as bad an economic time as now.
MARGARET WARNER: Next door, 75-year-old tailor Adnan Al Katami, who started in business with his father six decades ago.
Tourists from the region and abroad used to flock to buy Katami's handmade traditional Lebanese dress. Now he doesn't sell, so he can't buy either.
ADNAN AL KATAMI: I used to go shopping and bring things to the house when I finished the day. Now I can't do it because we are not making enough money.
MARGARET WARNER: Katami doesn't hold himself out as a soothsayer, but he sees a dark future ahead.
ADNAN AL KATAMI: It's going to spread in all Lebanon now, because the Sunnis are fighting and the Shia will start trouble in another area. So it's going to be bad, and there will be conflict and war, not only in Lebanon, but the region.
MARGARET WARNER: A Lebanon that looks like parts of Tripoli? Now, that would be a nightmare scenario.