As soon as word spread that President Trump had fired Gen. Timothy D. Haugh, the head of the National Security Agency and U.S. Cyber Command, current and former administration officials began floating theories about why he had been let go.

Had General Haugh opposed one of Mr. Trump’s initiatives, perhaps moved too slowly on purging officers who had worked on diversity issues? Or was he a casualty of the administration’s shifting priorities to counter narcotics?

Whether any of that was true, it had little, if anything, to do with why he was fired.

General Haugh was ousted because Laura Loomer, a far-right wing conspiracy theorist and Trump adviser, had accused him and his deputy of disloyalty, according to U.S. officials and Ms. Loomer’s social media post early Friday. He was one of several national security officials fired this past week on her advice.

“I predict you are going to see some nonsense statement about some policy difference or something General Haugh wasn’t doing, but we all know what happened,” said Senator Angus King, a Maine independent who is on the intelligence and armed services committees. “Laura Loomer said it. She is the one who told Trump to fire him.”

Senator Mitch McConnell, the Kentucky Republican and former majority leader, lamented that the Trump White House had ousted General Haugh and was appointing people to Pentagon posts who were skeptical of America’s engagement with allies and the world.

“If decades of experience in uniform isn’t enough to lead the N.S.A. but amateur isolationists can hold senior policy jobs at the Pentagon, then what exactly are the criteria for working on this administration’s national security staff?” Mr. McConnell said. “I can’t figure it out.”

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