How is the confrontation over the letter ironic?

The conflict of Raleigh disclosing the truth of Stanhope’s decline comes to a quick and unexpected end when Osborne reads the letter aloud. In an instance of situational irony, the letter contains nothing that should have made Stanhope concerned. Raleigh’s letter is respectful, and elides any details of Stanhope’s PTSD, choosing instead to focus on bolstering the image of Stanhope as a hero who keeps his subordinates’ spirits up.