They carried the common secret of cowardice barely restrained, the instinct to run or freeze or hide, and in many respects this was the heaviest burden of all, for it could never be put down, it required perfect balance and perfect posture. (The Things They Carried.77)
It's interesting that the soldiers' potential cowardice is a "common secret." It's something they all have, and that they know they all have, and that they know they all know that they have (lost yet?)—but they still don't talk about it. Their fear of weakness, be it physical or moral, is their "heaviest burden," and it's always on their minds.
On the Rainy River
I survived, but it's not a happy ending. I was a coward. I went to the war. (On the Rainy River.79)
Usually you'd think that cowards run from the war, but O'Brien calls himself out for going in the first place. He let his shame and fear win out over his principles. For O'Brien, the strong thing to do would have been to run away from the war and bear the censure. Instead, he was weak.
"Oh, Jesus," he said, and moaned, and tried to slide away and said, "Jesus, man, don't kill me." (Enemies.2)
Strunk and Jensen initially had a big tough-guy pact that if one of them ever got a wheelchair wound, the other would put him out of his misery. But when Strunk does get a wheelchair wound, he chickens out. Whether or not this should be considered weakness on his part is up for debate, though. On the one hand, Strunk sounds kind of pathetic and weak—he's swearing and moaning and begging. On the other hand, of course he sounds pathetic and weak. His leg just got blown off! And maybe choosing to live with the disability is the choice that shows strength.
The Man I Killed
I was terrified. There were no thoughts about killing. The grenade was to make him go away—just evaporate—and I leaned back and felt my mind go empty and then felt it fill up again. (The Man I Killed.4)
There's no way that the young man on the trail is a danger to O'Brien. He doesn't even know that O'Brien's there. O'Brien kills him not because of morality or politics, but because he's generally terrified and weak, and just wants the young man to go away. Again, we have a situation where the brave thing to do would be not to kill somebody.
Speaking of Courage
Sometimes, like that night in the s*** field, the difference between courage and cowardice was something small and stupid. The way the earth bubbled. And the smell. (Speaking of Courage.63-4)
It's kind of crazy to think that the difference between courage and cowardice could be something small. We usually think of the difference as a vast gulf; there are heroes, and there are weaklings. But Bowker says that something as simple as a smell can turn a hero into a coward. It's terrifying.
He wished he could have explained some of this. How he had been braver than he ever thought possible, but how he had not been so brave as he wanted to be. The distinction was important. (Speaking of Courage.126)
There's a tension here between what soldiers think of as brave and what non-soldiers think of as brave. While Bowker may have been pretty freakin' brave, he wasn't brave enough, and that's what kills him. That's what he can't explain.
In the Field
[…] maybe then he would write a letter to Kiowa's father. Or maybe not. Maybe he would just take a couple of practice swings and knock the ball down the middle […]. (In the Field.119)
This entire time, Jimmy Cross has been telling us that he completely accepts the blame for Kiowa's death, and that he's going to write Kiowa's father a letter explaining himself and accepting responsibility. At the end of the story, though, you know he's probably not going to actually do it.
When you give yourself an out on something that scary, you know you're going to take the easier option. And that shows an incredible amount of moral weakness on Cross's part. By chickening out from writing to Kiowa's father, he's also ducking the full weight of the responsibility for his death.
The Ghost Soldiers
[Azar] looked down at me with a mixture of contempt and pity. After a second he shook his head. "Man, I'll tell you something. You're a sorry, sorry case." I was trembling. I kept hugging myself, rocking, but I couldn't make it go away. (The Ghost Soldiers.204-5)
At first, Azar seems like the strong one here, and O'Brien comes across as weak. After all, Azar is pitying O'Brien, and O'Brien is a curled up mess on the ground. But O'Brien's the one with the strength to stare at his own weakness and mortality and deal with it (even if it involves crying). Azar's the one too weak to acknowledge that he could ever be weak.
Quietly, Sanders looked at me for a second and then walked away. I had to get Azar in on it. He didn't have Mitchell Sanders's intelligence, but he had a keener sense of justice. (The Ghost Soldiers.100-2)
Okay—if O'Brien is at the point where he thinks that Azar has a keen sense of justice, then he's really hit a moral low point. Instead of dealing with his feelings about Jorgenson and moving on, he's resorting to vengeance. Very weak.
He took off his boots and socks, laid out his medical kit, doped himself up, and put a round through his foot. Nobody blamed him, Sanders said. (Night Life.24-5)
When we learned that Rat was wounded in "The Ghost Soldiers," we assumed that it happened in battle. We'd never seen him be anything less than brave. That's why it hurts when we see him lose it and give in to weakness. If Rat loses it, then we could all lose it. And maybe it's true that nobody blamed him for punking out. Obviously, they've all punked out at one time or another. But Rat probably doesn't believe that nobody blames him, and he will live with that feeling of weakness forever.