Infantry and armor soldiers alone do virtually all the intimate killing. Here’s where the issue gets hard for me. Intimate killing is done in small units, normally squads and teams. In these engagements they fight and often die not for country or mission but for each other. We borrow a phrase from Shakespeare’s “Henry V” and term this phenomenon the “band of brothers effect.” This is the essential glue in military culture that causes a young man to sacrifice his life willingly so that his buddies might survive. Contemporary history suggests that U.S. infantry units fight equally well when made up of soldiers of different ethnicities, cultures, intelligence and social background. The evidence is also solid that gays make just as good infantrymen as do straight men.

I’ve been studying the band of brothers effect for almost 40 years and have written extensively on the subject. We know that time together allows effective pairings — or “battle buddies,” to use the common Army term. We know that four solid buddy pairings led by a sergeant compose a nine-man, battle-ready squad. The Marine squad is slightly larger. We know from watching Ranger and special forces training that buddy groups form often spontaneously. But the human formula that ensures successful buddy pairings is still a mystery, and that’s the key stumbling block in the debate. Veteran SEALs, special forces, Rangers, tankers and line infantrymen will swear that the deliberate, premeditated and brutal act of intimate killing is a male-only occupation. But no one can prove it with data from empirical tests because no such data exist from the United States. They just know intuitively from battlefield experience that it’s true.