War rarely yields any sort of moral certitude. For soldiers immersed in a climate where violence is not only condoned but mandated, the distinction between acts committed in the name of duty and decisions borne out of something darker can become dangerously blurred.

This seems to be the takeaway from Jim Frederick’s new work of narrative nonfiction, “Black Hearts.” Frederick initially intended to write about a prosecuted war crime in which four American soldiers raped an Iraqi girl and brutally murdered her family, an incident that drew international media coverage. But the work expanded to examine the circumstances preceding the horrific act, chronicling the steady unraveling of a platoon stationed in a particularly volatile section of southern Baghdad.

Frederick, a contributing editor at Time, spent hundreds of hours interviewing dozens of soldiers and relatives of the Iraqi family, culled information from court transcripts and internal Army reports, and spent several weeks embedded with a unit in 2008. What emerged, he said, was a complex narrative that, while culminating in an unambiguously immoral act, drew out the full spectrum of human behavior, from honor and loyalty to deepest depravity.

“All I ever tried to do was to understand the soldiers as human beings who make good and bad decisions, rather than good and bad people,” Frederick said in an interview. “It becomes much more complex and even in in-depth interviews there are human limits I can’t fathom.”

Frederick offers some broader perspective, including now well-worn critiques of the Bush administration’s failure to provide enough troops or to adequately anticipate the complexities of running a successful counterinsurgency campaign. The bulk of the book, however, is a reconstruction of the ordeals of American soldiers: their dialogue, a soldierly vernacular peppered with “dude” and liberally laced with profanity; the often-fraught relationship between officers and enlisted men; and battle scenes that spare none of the gory details.

For those who are fascinated with how soldiers live, “Black Hearts” will hold some appeal — it is clear that Frederick has done his homework and strove to create the most accurate possible picture. However, the endless procession of confusing missions, biographies and arguments between soldiers can become tedious. The reader begins to feel some of the same psychological grind as the soldiers.

While the book’s cast encompasses several dozen soldiers and various levels of the 101st hierarchy, the focus is primarily on the Bravo Company’s 1st Platoon, which as the book proceeds develops a reputation for recklessness, disorganization and substance abuse.

From the beginning, the soldiers of the 1st Platoon suffer from a lack of resources, a poorly defined strategy and faltering leadership. Their operations are run out of a large barn, they frequently lack equipment and are chronically short of troops. The result is grueling shifts without rest that stretch to several days and missions that seem irrationally dangerous.

“They did not have enough men and resources to accomplish the task that was handed to them,” Frederick said. “They were asked to do the impossible, and the United States military tends to believe that it can do the impossible. When you ask soldiers to outperform the long-term run of human endurance, those soldiers are going to break down.”

The platoon loses a disproportionate number of men, and as the casualties mount, the soldiers become less and less able to grasp the purpose of the hazardous missions they must perform with minimal support. A gulf opens between the men and a commander who is often abrasive, overly focused on formalities and prone to blame the soldiers’ incompetence for their comrades’ deaths. A rotating door of officers deprives them of any consistent, firm leadership.

“Isolated physically and with limited links to the outside world, Bravo soldiers frequently had no knowledge of how their efforts were fitting into the broader strategy of the war, let alone what that strategy might be,” Frederick writes. “Indeed, the very notion of strategy, and whether that strategy was sound, was simply not a concern for many of them.”

What results is a steady deterioration of discipline and morale, as the Bravo soldiers become increasingly isolated from any semblance of a coherent, overarching plan. A nihilistic attitude develops to the point that some soldiers even begin to hope for death — one of the soldiers is quoted as saying, “I knew I was going to die, it was just a matter of time, so I didn’t care.”

Alcohol and drugs, forbidden by Army protocol, become common currency. The platoon’s deterioration parallels a rising antipathy and mistrust of Iraqis, particularly after the deaths of two particularly well-liked soldiers.

Out of this brutality grows the horrifying crime that formed the initial impetus for Frederick to write the book. By the time the book reaches its climax, the motif of ubiquitous violence and apathy has become deeply established — the perpetrators already have a reputation for “getting drunk and going out looking for Iraqis to beat up,” Frederick writes.

Frederick’s reporting makes clear that, despite the lax morality pervading the life of the platoon, the slaughter of a family of Iraqi civilians was by no means inevitable. Private Steven Green, the ringleader, made no effort to conceal his xenophobic hatred and desire to kill Iraqis, even stating these tendencies explicitly to leaders and an Army psychiatrist.

Sgt. John Diem, consistently the most thoughtful and eloquent voice among the soldiers quoted in “Black Hearts,” suggests that the fractious and alienating military structure that allowed the platoon to descend into chaos was equally complicit in allowing four enlisted men to, for a time, get away with murder.

“It was the feeling of isolation at all levels of command that caused what happened,” he is quoted as saying. “If people continue to treat this like a mysterious event that came out of nowhere, and don’t change how we lead soldiers, and we don’t honestly look at what caused this to happen, it’s going to happen again. I mean, it’s already happened. This isn’t the only time. It’s just the most notorious time.”