Tonight, it is an uprising that started in the hopeful days of the arab spring last year, when tyrants from egypt to libya were being overthrown. But the tight in syria has dragged on and on and... See More
Tonight, it is an uprising that started in the hopeful days of the arab spring last year, when tyrants from egypt to libya were being overthrown. But the tight in syria has dragged on and on and spiraled into a brutal civil war. But now, it may be reaching a tipping point. Abc's alex marquardt traveled behind rebel lines to bring us this dramatic report from just outside aleppo, syria. Reporter: It's the most decisive battle of the 17-month long uprising. The fight for aleppo. Victory by rebels here could mean the end of the 40-year dictatorship of the family of president bashar al assad. The toll, brutal, for civilians and fighters on both sides. As the fighting intensifintensifies, we're invited to join a group of young rebels in a safe house just 20 miles away. It's kind of a last supper. Tonight, they're going to join the fight. Are you afraid? "No, no, no," says ali. "We're only afraid of god." They are all defectors of the syrian army. The mood is light. They joke. Mohamed tells us about his fiance. No sign they are nervous for the enemy waiting for them just down the road. Then, it's time to gear up. They eagerly wait for their names to be called. Weapons are handed out. AK-47s, ROBERT-PROPELLED Grenades. We go back to the home where a local family is putting us up. There, we immediate a young english teacher who says the government forces arrested him and for four days, he was tortured. They tell me -- day told me that they would let me die slowly. Like a dog. Or an animal, because I am not a human. Reporter: He was repeatedly hit with the butts of rye faifles. Simply, he says, because he lives in this town that is no longer controlled by assad forces. It is hard to describe it. Reporter: It's stories like his what have fueled what is clearly a civil war. Pitting syrian against syrian. So far, an estimated 20,000 people have been killed. But as in any conflict, the fog of war blurs lines. Here in syria, it's not just a civil war. We drive out of town, looking for a church where we've been told we can find foreign islamic militants. They've been streaming in from across the muslim world. Their goal? To help take down president assad and create an islamic state. As we get close, we run right into two cars full of them, heavily arm and angry to see us. So, we hide our camera. Well, that was interesting. We were stopped by a van full of what were clearly fundamentalist fighters. They stopped our car. They wanted to see our passports AND I.D.s. Clearly very unhappy to see us. It was quite interesting, they were speaking arabic with a non-syrian accent. We got out of there really fast. Even more sinister, and this is what really makes the united states nervous, al qaeda has joined the fight, carrying out a wave of attacks and suicide bombings. And this is a place where things are often not as simple as they seem. What happened to your neck? We meet this 8-year-old boy. He tells us the slashes on his throat come from assad shoulders who dragged him with a wire noose, trying to find out where his father was. It seems, given all we've seen and heard, a plausible story. But the next day, he breaks down in tears. He has a confession. He tells us he was told by his uncle to lie to us. His wounds, he says, come from an accident. His friends show us the weapons their little village has faced. Why are you holding onto this? "For the media," he says. We quickly learn it's a very ne line between civilian and rebel soldier. The morning after our last supper with the rebel fighters, we meet the fondo brothers. One they haven't worked their jobs in awhile, because now, their job is to fight and win a revolution. "We're proud to fight together," he sails, "fighting beside your older brother gives you more courage to fight." His son and another brother are currently fighting to take aleppo. Aren't you worried that four broerls fighting together, you could all die and leave your children without any fathers, wives would any husbands? "How can we see all of this happening and not fight for our dignity and freedom" he says. "We don't care if we die." Up on the roof of their home, this 7-year-old sings about battling the dictator. An act unthinkable just 18 months ago. But the war has changed everyone. When it started, they tell us, the little girl used to cry and hide in the bathroom as shells rained down on the town. Now, she comes up to the roof to watch where they're falling. For "nightline," I'm alex marquardt in northern syria.
This transcript has been automatically generated and may not be 100% accurate.