P.W. Singer once met a former child soldier who had been drawn into fighting at the age of 14. The teen, freed from his former life, was now giving speeches on the horrors of child warfare. Yet Singer detected some subtle aftereffects. At one point, the ex-soldier recalled a fight in a rehab hospital for child soldiers, in which kids who had been on different sides of the war whaled on one another with broken wooden chairs. Telling the story, Singer says, the speaker “got this humorous glint in his eye. He still saw the episode as a bit funny.”

In his second book, Children at War, the Brookings Institution scholar and director of the Project on U.S. Policy Towards the Islamic World offers a comprehensive look at a rarely covered aspect of contemporary war: the use of children as combatants. Children at War grew out of research Singer did for his first book, Corporate Warriors: The Rise of the Privatized Military Industry, the research for which brought him into contact with mercenaries and civilian contractors who had spent time in war zones all over the world—and encountered child soldiers almost everywhere they went.

The more he looked, the more Singer—a Princeton- and Harvard-trained scholar living in Arlington—discovered that child soldiering, a practice rare in modern history, had begun to increase in incidence during the past decade. Singer’s book, assembled from interviews with human-rights workers and recovering child soldiers, maps the unfortunate confluence of global events that have made widespread child soldiering possible.

The trend appears to have begun in the mid-’80s, when Iran’s theocratic rulers, locked in a bitter war with Iraq, sent entire brigades of children to slaughter. Within a few years, child soldiers as young as 5 (though more commonly between 8 and 18) began turning up in bloody civil wars in Africa and Asia. Now, in countries from Myanmar to Congo, from Liberia to Colombia, desperate poverty has been compounded by warlordism, drug trafficking, and the mass orphaning of children whose parents have died of AIDS.

Singer makes clear that child war is a far cry from playground cops-and-robbers. In many places, children are kidnapped and forced to kill under the threat of being killed themselves. Even when they enlist “voluntarily,” kids have few other options—nor the maturity to offer genuine consent.

“Usually it’s desperation. It’s the only way to get food or keep yourself safe,” says Singer. “Or it can happen if a family member is killed and you are honor-bound to take blood from the other side.” Adult leaders of child brigades frequently use drugs, psychological cruelty, or violence to keep their charges in line.

Singer concludes, depressingly, that child soldiering is a successful strategy. “It would not have spread if people didn’t consider it effective,” he says. Children can be relied upon to fight in wars that adult men aren’t willing to, and the inherent fearlessness of children makes them more willing to execute risky battlefield tactics, such as charging an enemy who is armed and firing.

At 30, Singer isn’t much older than some of the ex-soldiers he’s interviewed in eight countries over the past several years. This fact has added to the sadness inherent to his project. “Yeah, it depresses you,” he says, sighing. Studying child soldiering, he says, has made him “dark, moody, and gloomy. Sometimes it would bring a tear to my eye, or I’d just get really angry at something I shouldn’t be angry about, like when the refrigerator breaks.”

One of the keys to eliminating child soldiering, Singer argues, is to give adult commanders a reason to think twice before relying on children. Some shame is illustrated by a practice of the Liberation Tigers of Tamil Eelam, a Sri Lankan rebel group known for its coldly effective tactics. In the Tigers’ cemeteries, Singer notes, the children’s headstones do not list birth and death dates.

“They know what they’re doing is wrong,” he says. “They know history will judge them harshly.” —Louis Jacobson