I just watched an old fim called Sergeant Rutledge and confused by a line. The story happened in a court martial. After the judge advocate introduced himself, the president says: "Oh, yes. Lawyer, eh? Lawyer. Sent down here to keep a staff eye on field soldier justice, is that it?" The judge advocate replys: "On the contrary, sir, justice was guaranteed the accused when you were detailed as president of this court."
Could anyone explain the meaning of "justice was guaranteed the accused" here? Thank you!

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Justice was guaranteed the accused = The accused was guaranteed to receive justice  (as soon as that judge was appointed to the case).
The lawyer is flattering the judge.

It's a less common, slightly old-fashioned word order using passive voice.

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It’s an overly complicated phrasing lol but he is referring to the person who is on trial as “the accused” instead of calling him “the defendant”