The warrior's willingness to give his life for his fellow warriors.
Warrior altruism is an integral part of the Warrior Ethos: Manhood, Manliness, Courage and Valour; Justice, Wisdom, Faith and Fidelity; Self-Control and Self-Sacrifice; and Prowess in battle.
Warrior altruism is a particular manifestation of the trait sociobiologists call "reciprocal altruism," which can be described in somewhat simplified terms as: I'll do something for you if I'm relatively certain that you, at some point in the future, will do something for me.
Among those male-bonded groups of fighters we call warriors, reciprocal altruism takes an extreme, but very effective form: I'll die for you if you'll die for me.
The norm of warrior altruism, however, also manifests in non-lethal, non-combat, dyadic situations, and is what makes male-male couples who are emotionally and sexually faithful so powerful and so strongly bonded.
Because, ultimately, the men in these couples know they would die for each other.
Such bonds are honored in myth and epic -- Gilgamesh, The Iliad, The Celtic Tain; and in history -- David and Jonathan, Harmodius and Aristogeiton, the Sacred Band of Thebes, Alexander and Hephaestion.
See also Fidelity, Heroes, Heroic Homosex, Warrior Ethos, and Heroic Love.
And The Warrior Altruism of the Warrior God.