Why Do Soldiers Fight?

In October 1862 a Union soldier named Hillory Shifflet wrote his wife with a sobering account of his experiences in battle. It was a description riddled with near-fatal episodes. Rebel bullets had struck a neighbor in the ranks—the man was "shot through the heart and dide instantly"—and Shifflet, who was "right clos to him," experienced a number of harrowing close calls. Confederate balls had shot his gun barrel off at the breech and his cartridge box off at his belt; even more disturbing were the four bullet holes Shifflet sustained through his shirt. The brushes with death were traumatic enough that he briefly left the battlefield. But despite the fear and tangible dangers of the firing line, Shifflet soon obtained another musket and returned to combat even as he acknowledged that the enemy projectiles "will never come enny nerer with out kilen me."

How did he marshal the will to return? What made Shifflet, and generations of soldiers before and since, stifle powerful instincts for self-preservation and summon the resolve to kill other humans amid the chaos and trauma of the battlefield? What, in short, makes soldiers fight? That deceptively straightforward question has bedeviled armies for millennia. It also represents one of the central mysteries of human behavior: for the individual soldier, participation in combat demands actions that run counter to hard-wired natural drives to avoid danger. Psychiatrists studying American aircrews during the Second World War reduced the question to its bare essence: What, they asked, can possess a rational man to make him act so irrationally?

The question has important implications not just for military systems but for our broader understanding of human nature. Generations of scholars and soldiers have applied a variety of tools and methods in the effort to explain the seemingly inexplicable. The question became even more difficult in the 20th century as changes in the nature of combat meant that armies could no longer employ direct physical coercion to drive troops into battle. In the years immediately following the Second World War the study of soldiers' experiences in battle yielded a new formulation to explain the seemingly irrational behaviors observed on the battlefields of that war. That hypothesis posited a group of pressures and inducements that revolved around the desire for acceptance from some valued social group—a potent blend of friendship, concern for reputation, mutual interdependence, and trust that formed among members of a combat unit. The power of those emotional bonds, scholars argued, could eclipse an individual soldier's own instincts for survival, if only for a time.

Freeman Mason of Company K, 17th Vermont Infantry, holding a tintype of his brother, Michael Mason, killed at Savage's Station, Virginia, in 1862. Library of Congress, Prints and Photographs Division [reproduction number, LC-DIG-ppmsca-37071].

A trio of influential studies that appeared in the years following World War II coined the term "primary-group cohesion" to describe the power of the emotional ties that formed among members of close-knit military organization, and scholars credited those cohesive bonds with the ability to get soldiers to overcome their fears and fight despite nearly overwhelming reservations. S.L.A. Marshall, author of one of the most prominent of these studies, defined the relationship between group-cohesion and the motivation to fight plainly. It was "one of the simplest truths of war," he wrote, "that the thing which enables the infantry soldier to keep going . . . is the presence or presumed presence of a comrade." Nonspecialists often recognize the group cohesion thesis as the "band of brothers" phenomenon, an explanation for soldiers' willingness to endure the trauma of battle that takes its name from the St. Crispin's Day speech in Shakespeare's Henry V. Like Prince Hal, the thesis envisages soldiers as a "band of brothers" united by their common experiences and sacrifices: a group whose affection, obligation, and loyalty enable its members to overcome individual fears and concerns in the cauldron of battle. At bottom, it argues, soldiers fight primarily for one another.

It has proven an enormously powerful hypothesis. In the decades since the Second World...