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Hennessy was the only one that ever stood up for Arnold and me. That fact didn't occur to me until I saw him being hauled off to prison.
— Jerome • Biloxi Blues

Behind the line May contain spoilers.
Our reading
An editorial interpretation.
The speaker recognises a defender only while watching that person being taken to prison. The belated realisation makes the support seem especially poignant: someone had stood up for both people without receiving this recognition. The passage connects gratitude with the sting of understanding another person's importance too late.
Context reviewed