The federal judge overseeing former President Donald J. Trump’s classified documents case struck down a small but significant part of the indictment on Monday, ruling that the government must remove from its charges an episode in which Mr. Trump is said to have shown a highly sensitive military map to one of his close aides after he left office.

The decision by the judge, Aileen M. Cannon, was more of a swipe at prosecutors working for the special counsel, Jack Smith, who brought the case against Mr. Smith, than a major blow to the allegations in the case. Even though Judge Cannon technically removed the incident from the charges, prosecutors may still be able to introduce evidence of it to the jury if and when the case finally goes to trial.

The incident that Judge Cannon struck took place in August or September 2021 at a meeting at Mr. Trump’s golf course in Bedminster, N.J. During the meeting, prosecutors say, Mr. Trump showed a classified map related to an ongoing military operation to a representative of his political action committee, widely believed to be Susie Wiles, who is now a top adviser to Mr. Trump’s campaign.

As he displayed the map, prosecutors say, Mr. Trump told Ms. Wiles that the military campaign was not going well.

Lawyers for Mr. Trump had challenged the inclusion of that episode, among others, in the indictment as part of a broader attack on the charges, saying it was extraneous and irrelevant. They argued that it should never have appeared as part of the charges because Mr. Trump has not been formally accused of improperly transmitting classified material to other people, only with illegally retaining it after he left the White House.

In her ruling, Judge Cannon denied the lawyers’ request to dismiss the charges altogether, but noted that prosecutors had elected to charge Mr. Trump in the case with what is known as a “speaking indictment” — one that describes events in evocative language rather than merely listing dry violations of the law.