From Boot by Daniel Da Cruz (my italics):

Esprit de Corps

The intense esprit de corps that exists among grunts is one of the main appeals of the fleet Marine Force to recruits. A cement which binds together men facing a common danger, it is often stronger than that most primeval of human drives, the instinct for self-preservation.

Esprit de corps, the consciousness of and pride in belonging to a particular group, the sense of shared purpose and fellowship, ideally should apply to every military unit. To the degree that it does, that unit will distinguish itself, especially in combat, from others not similarly animated.

That conclusion, demonstrated with monotonous regularity during millennia of combat, stems from the objectives for which men fight. Contradicting the popular view, men fight first - not for flag or fatherland, but for the sake and respect of buddies in their squad. The next tier of loyalty is to their company (or ship), the third to their branch of service, and only then come the more nebulous considerations of flag and nation.

The British have profited from an understanding of this reality at least as far back as the sixteenth century, when Shakespeare spoke of the solidarity of "We few, we happy few, we band of brothers." Officers and men do not join the army as such, but individual regiments in which men serve their entire careers, promoting the strongest bonds of mutual loyalty. The Coldstream Guards, Honorable Artillery Company, Gurkhas, and the Black Watch are but a few of the units made famous by fighters fierce in fidelity to their fellows. At the other end of the scale are many infantry units of the U.S. Army, in which men are replaced like spare parts on an automobile, with consequent lackluster performance in battle. After all, who willingly and habitually risks his life for strangers?