One used to need strength and skill to fight in a war. Armor and lances are heavy; swords, knives, and bayonets require study. But now war is easy. A child can blow away dozens of people with a 10-pound, automatic-firing AK-47. These Soviet weapons, which cost less in Africa than a movie ticket costs in America, can be mastered by anyone in 30 minutes. This evolution in weaponry is one of the principal reasons why children have come to comprise a disquieting percentage of the world’s soldiers, writes P.W. Singer in his new book, “Children at War” (Pantheon, 288 pages, $25) Often kidnapped, often drugged, often cannon fodder, children now comprise about 10% of soldiers currently engaged in combat.

There were almost no child soldiers just a few decades ago, but now 5-year olds fight in East Africa, 12-year olds smoke cigars and raid hospitals in Southeast Asia, and Hamas convinces 16-year old semi-retarded children to strap bombs to their bellies. Another reason for this modern humanitarian crisis is that war is less ordered than in the past. Communists aren’t fighting democrats over political power on battlefields with specific rules. More likely, different gangs are fighting each other over diamonds, meaning there is little outside pressures that can halt child conscription.

In other cases, the battle is so unequal that the weaker side throws out the rules and norms of war that have kept children off battlefields. More than 20% of the Palestinian killed in the Intifada have been 17 or under, and Palestinian armed groups seem set on continuing that trend, because children have a much easier time crossing borders and avoiding searches. The Palestinian equivalent of “Sesame Street,” called “Children’s Club,” once included a song with the lyric “when I wander into Jerusalem, I will become a suicide bomber.”

The results of the mass armed mobilization of people who should be playing with soccer balls (or teddy bears) is tragic, and Mr. Singer details the carnage well. Children don’t understand death and are easily indoctrinated, so they take grave risks and are often sent to the front lines or asked to serve as human minesweepers. If they don’t die, they suffer. Children’s limbs grow faster than their surrounding tissue, and so children who lose limbs in battle need frequent additional amputations. In the best case scenario, the battle ends, peace returns, and they try to live as adults despite having spent their formative years butchering and raping their countrymen.

Mr. Singer is a fellow at the Brookings Institution and seeks to both describe the epidemic and to solve it. The United States worked to weaken the 2001 convention on the international small-arms trade, for example opposing a system that would facilitate the tracking of small weapons internationally. Mr. Singer says that we should have done the opposite. He also recommends that concerned people spend less time writing new laws proscribing the use of children in warfare and more time enforcing the many laws on the books.

He fingers one of the main problems in international law when he recommends that activist groups throttle their animus toward the United States. The Coalition to Stop the Use of Child Soldiers has spent much of the past few years castigating the United States for allowing 17-year-olds to enter the American and British armed forces with parental consent. If that’s a sin, it’s on a much lower scale than judging that people are ready to fight once their molars have come in.

Worse than just being foolish, this sort of campaign further convinces American policymakers that international law is a mechanism for pony tailed European activists to torment the United States – not one of the principal solutions to the problem.

Mr. Singer does stretch too far, calling for an end to all recognition of and trade with groups that use child soldiers. This sounds noble, but it’s impractical. Concern about child soldiers can’t be one’s first principle when it comes to something as complex as war. In Eastern Burma, for example, the oppressive and brutal Burmese junta has carried out a long-term slaughter of an ethnic group called the Karen. The United States for humanitarian and strategic reasons should clearly support the ethnic minorities in this conflict, but the Karen use child soldiers in their insurgency. We should increase military aid to them and use that as leverage to convince them not to send children into battle.

Overall, Mr. Singer has strong proposals. The book’s largest problem is that he lapses into academic jargon (particularly while offering policy proposals) and focuses far too much on minute details, like specific cleansing ceremonies that can help child soldiers reintegrate into their communities. But “Children at War” is a brave and important book, and an impressive cataloging of a horrible, and surprisingly modern, problem.

Mr. Thompson is a senior editor at Legal Affairs.