The slaying this month of Manuel Jesús Muñoz, a member of the ruling directorate of the Revolutionary Armed Forces of Colombia, was a dramatic signal that a rebel group known for its resilience is engulfed in an internal crisis that could lead to its implosion after four decades of armed struggle.

In a country where most people cannot remember a time of peace, Colombians are for the first time raising the possibility that a guerrilla group once thought invincible could be forced into peace negotiations or even defeated militarily.

Weakened by infiltrators and facing constant combat and aerial bombardment, the insurgency is losing members in record numbers. The FARC, as the group is known, lost 1,583 fighters in combat last year, its columns are plagued by command-and-control problems, and popular support is evaporating, the government of President Álvaro Uribe says.

Since 2000, the Uribe administration has received $5 billion in U.S. aid, mostly for military and anti-drug programs -- more than any other government outside the Middle East. The money has helped it revamp the Colombian army, paying for new helicopters and training for elite troops, although rights groups remain concerned about abuses, including the killings of civilians.

The most serious problem the FARC is facing is not guerrilla deaths or the loss of territory, but mass desertion, according to political analysts, military officials and former guerrillas interviewed this month. Many said desertions have badly hurt morale and provided the military with important strategic information about the hermetic group.

"If the situation continues like this, the FARC will be finished," said Ivan, 33, who deserted from the group Dec. 27 after serving as the No. 2 commander of a unit in the coffee-growing west.

"It won't be tomorrow, and it could take years, but it will happen," said Ivan, who asked that his last name not be used out of fear the FARC might kill him.

Montoya, 33, said the threat comes from men like him who have lost their devotion to the FARC. In an interview at a military base here, he spoke of how he had once dreamed of taking power and spent 16 years becoming an experienced warrior. But then, he said, he began to fear being killed by the Colombian troops chasing his rebel unit or even in one of the purges his commander had been ordering.

"The internal situation is this: Finances, bad; medicines, bad; supplies, bad," Montoya said.

"The civilian population does not want to collaborate," he added. "There is a complete rejection. Even civilians are telling guerrillas: "Desert. Don't let yourself get killed."

Last year, 2,480 rebels abandoned the FARC, up from 1,558 the previous year, according to the Defense Ministry. Most are classified as "men in arms," fighters on the front lines of a simmering conflict. About 40 percent are plainclothes militiamen who carry out intelligence operations and supply provisions to FARC units.

In interviews in government-run safe houses, deserters from eight different "fronts," as FARC units are called, talked of a rebel organization unable to supply itself adequately or coordinate among far-flung fighters. The former rebels recounted the sheer terror of being bombed by Colombian fighter planes and the constant firefights with troops.

They also described the hardship of being in units that have lost half or more of their fighters in the past five years, some to combat but most to desertions. Montoya estimates that his old unit was down to about 60 fighters, far from the 250 it had in 2002.

"It's massive, from commanders to low-level guerrillas," Montoya said of the desertions. "People are tired."

The deserters also say that increasingly paranoid commanders have started to stage more "war councils," jungle trials in which guerrillas face execution if found guilty of treachery.

Montoya said that Muñoz, his column harried by army troops and lacking food and ammunition, had held several war councils. The commander's computer, which Montoya turned over to the army, contained proof of dozens of trials.

"Anything you do that generates a lack of confidence, they call a war council," said Rocio, 28, who deserted Dec. 14 from the 40th Front in the plains state of Meta. "They prefer to liquidate than to have you run away."

The assessment comes not just from soldiers and deserters, but also from leftist opponents of the Uribe administration.

"What the guerrillas need to understand is that today, in the 21st century, after so many things have happened in the world, the armed revolutionary struggle is not an option," said Carlos Lozano, editor of the communist weekly Voz and a member of the Communist Party, which has historical ties to the FARC. Lozano and other analysts, including some government officials, say the FARC does not face defeat anytime soon. The rebels remain entrenched in the country's vast southeastern plains and jungles and are increasingly active along its poorly patrolled borders. The group is also believed to continue to draw a steady tax from the cocaine trade.

"The FARC has suffered a lot, but they are still very powerful," said Rafael Pardo, a former senator, defense minister and author of books on Colombia's conflict. He noted that the FARC, while losing members to combat and desertions, has an "immense capacity to recruit" young, poor farmers with few other options.

But the FARC has slowly contracted from its zenith in 2002. The Defense Ministry said the group is estimated to have fewer than 11,000 fighters, down from 16,900 in 2002.

In the past few months, the armed forces have killed seasoned mid-level commanders such as Gustavo Rueda, who led a once-feared unit in the strategically located Maria Mountains of northern Colombia, and Tomás Medina Caracas, who ran the all-important 16th Front in the southeast, raising funds from the cocaine trade.

"It was huge when he died," said Angeli, 23, who joined the FARC in 2002 and deserted last year from Medina's unit. "He was the commander most able to motivate the young ones. The desertions began right after he was killed."

On March 1, Luis Edgar Devia, better known as Raúl Reyes, died when Colombian air force fighters bombed his camp just inside Ecuador. He became the first member of the FARC's seven-member secretariat to be killed. A week later, Montoya shot Muñoz.

Military officials and deserters say information gleaned from deserters helped the army in several successful operations against top commanders. "Desertion is much more useful, because you get information from a deserter," said Vice Defense Minister Sergio Jaramillo. "You hit the morale of the FARC much more than you do with a death in combat."

Deserting guerrillas are motivated by a range of factors, from the sense that theirs is a lost cause to the lure of government programs, advertised on national television and regional radio stations, that provide shelter, food and a stipend to deserters.

Although those accused of atrocities face trials, rank-and-file rebels are pardoned for the crime of "rebellion." In the case of Muñoz, the government says Montoya and other guerrillas who provided intelligence about the commander will share a reward of about $2.5 million.

Johana, who joined the rebels in 2003 when she was 13 and had a baby with a commander, described her three years in the FARC as being "dead." She was wounded twice and watched as droves of her comrades deserted, including her father and brother.

Since deserting from the 32nd Front in war-torn Putumayo state on Dec. 16, 2006, she has focused on her family.

"I want to be someone in life," said Johana, who is now 18. "I got this opportunity, and made it out of the guerrillas alive."