Question

In “Ambush,” which line signals the beginning of the narrative within the frame story?
When she was nine, my daughter Kathleen asked if I had ever anyone.
“You keep writing these war stories,” she said, “so I guess you must’ve somebody.”
It was a difficult moment, but I did what seemed right, which was to say, “Of course not,” and then to take her onto my lap and hold her for a while.
He was a short, slender young man of about twenty.

Answer 1

The line "He was a short, slender young man of about twenty." — it marks the shift from the present-frame (Kathleen asking) into the wartime narrative/flashback.