When Editha goes to visit George's mother after his death in the war, she tells the old woman that she would rather have died herself than send George to war to die. She starts to say that she tried to leave George free to make up his own mind about going to war, but George's mother cuts her off. Her words and tone are bitterly sarcastic:
Yes, that letter of yours, that came back with his other things, left him free.
Editha knows what she had written, but she can't believe it was her letter than sent him to war. She, after all, had told him not to open it until he was actually in battle. The truth, however, that George's mother knows is that her son surely read Editha's letter before going to war and that it was her letter that compelled him to go. She tells Editha:
Of course, he wouldn't read a letter of yours, under the circumstances, till he thought you wanted him to.
Again, her response to Editha is bitter in its sarcasm.
A careful reading of the story strongly implies that George's mother is right in assessing why her pacifist son enlisted. George had been a soft, gentle young man who had grown up in a home that abhorred war, largely as a result of his father's service and wounding in the Civil War. (His father had lost an arm in battle.) After spending time with Editha who put enormous pressure on him to enlist and reacts coldly to his own feelings about the matter, George goes to a rally intending to dampen the war fever; instead, he gets drunk, finds himself leading the cries for engagement, and becomes the first in the crowd of young men to enlist. Leaving the rally, he returns to Editha (still in a fever and still inebriated) to tell her what he has done. She gives him her letter and his reading instructions as he is leaving.
The next day, after he has sobered up, George comes back to Editha, and their conversation strongly implies that he has already read her letter. Editha tells George she knows he has enlisted "from the highest motives," and she knows he had not done it "for my sake" because "I couldn't respect you if you had." George replies: "Well, then we'll say I haven't." Clearly, it is Editha's letter, promising to break their engagement if he does not fight, that keeps George headed to war after his drunken act the previous night. The strong suggestion is that were it not for Editha's letter, he might have changed his mind. Her letter, however, has made that impossible since he loves her.
Sarcasm (also known as verbal irony) is defined as saying the opposite of what one means. When George's mother says Editha's letter set her son "free," she is being doubly ironic. The letter did exactly the opposite; it took away George's emotional freedom to make up his own mind about enlisting. Her words imply a deeper irony, as well. The letter set George "free" in the sense that he was now free of life's constraints: He was dead.