GWEN IFILL: Finally tonight, a look at one element of Syria's opposition, the Free Syrian Army. Many are defectors from Syria's regular army. That force is smothering the anti-Assad rebellion with brutal efficiency.
Hugh Macleod and Annasofie Flamand, a reporting team from our international news website partners GlobalPost, spent time with the fighters in northern Lebanon.
An unnamed photographer accompanied them into Syria.
Macleod voiced this report.
MAN (through translator): These guys with me are from the Free Syrian Army. They are defected soldiers from the army of the Syrian regime.
MAN (through translator): I feel proud that we, the Free Syrian Army, can say that we are here, that we are here on the ground. And, hopefully, soon, we will conduct large operations, operations to topple the regime.
HUGH MACLEOD, GlobalPost: One year into his brutal crackdown on peaceful protests, Syrian President Bashar al-Assad is facing an armed insurgency by the rebel fighters of the Free Syrian Army.
The group is led by officers and soldiers who defected from Assad's security forces after witnessing atrocities against their own people.
COMMANDER ABU ALI, Free Syrian Army (through translator): It was like a genocide against the people, and I was part of it. The army went to the streets and shelled houses with tanks and the demonstrations.
HUGH MACLEOD: Abu Ali is a field commander for one of hundreds of units fighting government forces across southern and central Syria. We've agreed not to reveal his full name.
COMMANDER ABU ALI (through translator): We will enter from this position. We can cross from here.
HUGH MACLEOD: An estimated 7,000 to 10,000 men are currently fighting with the FSA. Though Assad's forces outnumber and outgun the poorly armed rebels, scores of soldiers are still defecting every month to join the fight.
MOHAMMED, Free Syrian Army (through translator): For me, my conscience didn't allow me to watch as the regular army -- or, rather, there's nothing we call the national army. There's Assad's brigades and Assad's gangs, beat the people and commit crimes against humanity. There was a tank and around it there were soldiers. I stood here, away from it.
HUGH MACLEOD: Twenty-one-year-old Mohammed defected late last year and joined Abu Ali's unit. He tells his officer about firing a rocket-propelled grenade at one of the regime's tanks.
MOHAMMED (through translator): I stood here. Of course, the first rocket didn't work. I took it out and changed it. Then I launched the second one. That's when they were firing at me.
MAN (through translator): That's you?
MOHAMMED (through translator): That's me. I was holding the rocket- propelled grenade launcher. I fired it here.
MAN (through translator): Did you hit your target?
MOHAMMED (through translator): Of course.
HUGH MACLEOD: In the long hours between missions, the young fighters tried to keep up morale by remembering the friends and family they're fighting for.
MOHAMMED (through translator): "I miss you," it says. I miss my mother a lot. If I don't go back, I cannot see her or my family or anybody else. Of course, the injustice will not stop unless me and my brother here fight for it. I am supposed to call him "sir," but now I call him brother, because we fight for the same cause.
HUGH MACLEOD: In the last few weeks, FSA units have achieved their greatest successes, but also suffered their biggest defeats. Units in Homs were forced to retreat after battling a month-long government assault that killed at least 700 and wounded 2,000, mostly civilians.
But Abu Ali says his unit successfully captured two of the regime's key bases in Kasaya, a town 20 miles southwest of Homs.
MAN: Are you ready to fight with Shabiha and Assad's troops?
MAN (through translator): Yes, for sure. We can easily clash with them despite our lack of ammunition.
COMMANDER ABU ALI (through translator): We ask the international community to arm the Free Syrian Army, because we have only a few arms and even less ammunition.
HUGH MACLEOD: Their victory in Kasaya has come at a great cost. Abu Ali was wounded. And of the dozen fighters in his unit three, including Mohammed, have been killed.
COMMANDER ABU ALI (through translator): They are stronger with their deception, with their weapons, their tanks and their air force. But we have a stronger will, a stronger doctrine and stronger people. We fight for God. They fight for Bashar. We will see who will win in the end.