developments tonight approximate the u.N. Denouncing his deadly crackdown and demanding thck down of serious chemical and biological weapons. Aleppo is bracing for what both sides are now calling the... See More
developments tonight approximate the u.N. Denouncing his deadly crackdown and demanding thck down of serious chemical and biological weapons. Aleppo is bracing for what both sides are now calling the decisive battle of the war. Alex marquardt make the dangerous journey into syria with the rebels. Tonight they're on the move inside syria. Alex? Reporter: David, we're about to cross the border into turkey from northern syria, where we've just spent the past week meeting and staying with families and fighters in the middle of this conflict. We spent the night with one unit as they headed into what could be one of the biggest fights of their young lives. These are the no the faces of battle-hardened warriors. They are young men, barely 20 years old. Defectors from the syrian military, with just basic training. They call themselves "the martyrs of freedom." They tell us they can't wait to in the fight. They're going to aleppo tonight, the battle that assad said will determine the fate of syria. Reporter: Are any of you afraid about going to fight tonight? No, no, no, they say. We're only afraid of allah. Reporter: Allah, their god, and their inspiration. What these young men lack in battlefield experience, they make up for with hatred of the syrian regime, and religious fervor. The regime has tanks, planes, helicopters. YOU JUST HAVE AK-47s AND RPGs. How can you defeat them in aleppo? We have allah with us. He will send soldiers. Reporter: This is the holy month of ramadan when they fast all day. After sundown, they break the fast. The mood is light, not what you'd expect hours before the big fight. Then, it's time to gear up. They eagerly wait for their names to be called. WEAPONS ARE ASSIGNED, AK-47s, Rocket-propelled grenades. If they're nervous, they're hiding it well. Victory or marred ir dom, good willing says ali. They say goodbye to their brothers in arms, and head out into the dark. Those left behind, are disappointed. Abdulrahman went last time. Tonight, it clearly pains him to be missing out. For many this will be the first, and maybe last, fight of their lives. But there is not a hint of doubt among them. That this is a battle that must be waged, and won. Alex, our thanks to you and your team on the ground.
This transcript has been automatically generated and may not be 100% accurate.