Chamberlain thought: two things an officer must do, to lead men. This from old Ames, who never cared about love: You must care for your men's welfare. You must show physical courage. (2.4.66)
"Physical courage" is important for an officer for an obvious reason: if you wimp out, it's not going to set a great example for your soldiers, and if you crack under pressure, you'll probably get them killed. But it turns out that physical courage isn't enough. You need to have love and compassion for people; otherwise, there's no real motive for showing courage.
Hold to the last. To the last what? Exercise in rhetoric. Last man? Last shell? Last foot of ground? Last Reb? (3.4.57)
Chamberlain doesn't know how far he's supposed to go in holding his ground. Facing the test, he ends up going pretty far, indeed. Does his uncertainty about what is wanted make him act more decisively than he otherwise would have?
Chamberlain raised his saber, let loose the shout that was the greatest sound he could make, boiling the yell up from his chest: Fix bayonets! Charge! Fix bayonets! Charge! Fix bayonets! Charge! He leaped down from the boulder, still screaming, his voice beginning to crack and give, and all around him his men were roaring animal screams, and he saw the whole Regiment rising and pouring over the wall and beginning to bound down through the dark bushes, over the dead and dying and wounded, hats coming off, hair flying, mouths making sounds, one man firing as he ran, the last bullet, last round. (3.4.204)
Running out of ammo and determined to repulse the rebel attack, Chamberlain orders a bayonet charge. This turns his defense into offense—it's such an unexpected and daring move that it turns the Confederates back and helps win the battle—and maybe even the war.
Rice nodded. "So. You fixed bayonets.'
It seemed logical enough. It was beginning to dawn on him that what he had done might be considered unusual. He said, "There didn't seem to be any alternative." (3.4.286)
When he was in the heat of battle, Chamberlain didn't realize how courageous his actions were. He was so totally in the moment, so engaged with fighting, that he simply wanted to do what was needed. Do you have to know you're being courageous for it to count as courage? Or is courage more about being in the moment?