Continuing this discussion-- why the LEG 🦵 specifically?
I see how soldiers might target his leg symbolically if they accused him of cowardice and running from a battle. It would make sense that Pierce's sense of pride would want to hide that. However, I don't know that soldiers would attack him for it, nor that people would look down on him for it; if he were actively serving in the military, yes, that's desertion/cowardice & shameful and would risk retaliation. But he's observing the war as a civilian. He doesn't have any military issued weaponry to protect himself should he somehow get caught up in the action. I don't think anyone would have expected him to stand and fight in that scenario–with what weapon? Also, this wouldn’t have been his first experience of conflict, why would he have run from this one?
I have some slightly more specific hypotheticals working from my supposition–Pierce acted ethically, soldiers took it as personal offense or treason–to explain an injury directed at the leg, AND have it be something that Pierce doesn't like to talk about.
Firstly, the choice of his leg could mean nothing; it could only mean that the soldiers wanted to maim him because killing would be too risky (what to do with the body, what to do with his equipment, people are going to wonder where the only photographer went), and the leg is a large target with some chance of recovery. Furthermore, if he was in a fistfight (Islington does note that Pierce has a long reach and seems an experienced fighter pre-injury), his arms up to defend his upper body would make his legs an easier target. Then stabbing his leg might not have meaningful connection to the reason for the attack but simply have been a matter of access. It could also be they were trying to kill him, either by leaving him without mobility & abandoning him to die, or by hoping to cut an artery, causing Pierce to bleed out; either option could reasonably be blamed on a battle–struck by shrapnel, bad luck *shrug*
If the leg is symbolically connected to The Incident, I imagine a few scenarios in which they might have targeted his leg:
Fearing for how they will be treated, Pierce allows some POWs to escape. Soldiers are angry & decide to punish his legs for allowing someone else to "run."
Pierce catches a soldier doing something cruel - ex: harassing locals or attempting to rape a woman - and fights the man. In the course of the fight, he breaks the man's leg. His friends retaliate by cornering Pierce and injuring his leg.
Similar to above but Pierce interrupts/ catches the whole group doing something they shouldn't. They know he has witnessed this & see him as a threat, so they attack. They may have intended to kill him but were interrupted.
Pierce catches one or the group doing something unethical and openly threatens to tell their commanding officer. They set upon him and target his leg as a message to say "let's see how fast you can run and tell on us now. If you snitch, we'll do worse” or “hope you can make it back to camp, bye”
Perhaps Pierce does report one or more soldiers, leading one (or the group) to get in trouble & they hold a grudge over it. Their revenge is the maim-to-plausible-deniability-of-a-kill method.
Any of the above scenarios strike me as believable the soldiers would carry grudges and attack again, especially with a hefty dose of alcohol.
In the scenarios where soldiers turn on Pierce for protecting someone, they may still exact their violence against the innocent(s) after attacking him. Pierce's leg then becomes a constant source of internalised shame, a reminder that his intervention wasn't enough; that he wasn't enough. In many of these, Pierce would not want to talk about The Incident because:
if he was unable to protect innocents & his effort was for naught, he would feel regret and self-disparagement.
if his actions were technically treasonous, he would not want to endanger himself nor a loved one with the knowledge
if the attack was a warning meant to silence him, he may feel guilt and shame that he did not speak up
if Pierce's speaking up or reporting an unethical soldier(s) caused them to exact revenge on someone else (in addition to him), he would feel guilt and regret.
Another way to think of the injury: What if the leg is not connected to The Incident but rather The Growth? Pierce has had to rely on no one but himself and stand on his own two feet for a long time. What if the injury to his leg is more symbolic of him having to learn to lean on others? Now he’s more susceptible to being off-balance and might need a little extra support. Perhaps the injury is a consequence of his determination to conquer a problem independently, and through the injury he is learning how to let others in and support him?
What we know about Pierce as a character is that he has a low image of his self-worth. He tells Emma he knows she could be with other, “finer gentlemen” and that he is far below her. Emma sees him blaming himself even for some of the "catastrophes" of volume three:
"You seem cursed with our strange catastrophes."
Mr. Pierce looked thoughtful, rubbing the inside of his forefinger along his chin.
"Ironically, the events seem to trace back to Tybalt, the cat I introduced to Lapis Lazuli."
He was blaming himself. A ridiculous thing I could not countenance.
"It is not your fault," I replied. “A strange accident, as you said yourself.”
"We cannot always predict the outcomes of our actions, Miss Lion, however well intended."
That last line especially resonated in my research on this question. Could this reflect some of the situation which got his leg injured? A well-intentioned action that backfired?
Factoring in Pierce’s self-deprecation makes me suspect that The Incident is not one Pierce keeps quiet for fear of ruin or censure but because he unjustly blames himself or carries guilt or regret on behalf of others.