“I cannot be part of the same organization, even at a remove,” he wrote.
Fox News responded on Tuesday by saying it was “extremely proud of our top-rated prime-time hosts and all of our opinion programming.”
“Ralph Peters is entitled to his opinion despite the fact that he’s choosing to use it as a weapon in order to gain attention,” the network said in a statement.
Colonel Peters, who appeared regularly on Fox News and the Fox Business Network — including as recently as Monday morning — spent more than two decades in the Army, eventually specializing in Russian intelligence. He began appearing as a television commentator in the late 1990s, and signed an exclusive contract with Fox in 2008.
Typically hawkish in his views, Colonel Peters supported the invasion of Iraq in 2003 and was a strong proponent of confronting President Vladimir Putin of Russia. He was a fervent critic of former President Barack Obama, deriding his foreign policy as weak, and was briefly suspended by the network in 2015 after using a vulgarity to describe Mr. Obama during an appearance on Fox Business.
In an email interview on Tuesday, Colonel Peters wrote that his letter had been “intended for internal consumption,” adding: “I am not trying to grandstand. Fox was good to me for many years.”
He said he informed Fox News on March 1 that he did not plan to renew his contract, which expires toward the end of this month.
“As a retired military officer,” he said, “I simply could not continue with Fox in good conscience.”
Fox News’s commentary shows, like “Hannity” and “Fox & Friends,” have become stalwart defenders of Mr. Trump and his administration, often criticizing Mr. Mueller, the special counsel, and law enforcement agencies that have been investigating possible ties between the president’s associates and Russian interference into the 2016 election.
Fox News remains the No. 1 cable news network despite recent changes in its prime-time lineup. Since 2017, the conservative pundits Tucker Carlson and Laura Ingraham have replaced Megyn Kelly, who left for NBC News, and Bill O’Reilly, who was fired after a harassment scandal.
Colonel Peters’s message to his colleagues, which was first reported by BuzzFeed News, included the caveat that not every Fox News host is “a propaganda mouthpiece — some have shown courage.” Colonel Peters also described his respect for colleagues at the Fox Business Network and reporters at Fox News, whom he called “talented professionals in a poisoned environment.”
An author of historical fiction and spy thrillers who writes a regular column in The New York Post, Colonel Peters gained notice in recent weeks after he called for an assault weapons ban in the wake of the school shooting in Parkland, Fla.