For 10 years, Ralph Peters regularly appeared on Fox News to offer military analysis and insight as one of the cable network’s reliably conservative commenters. But he quit in March in disgust.

Mr. Peters, who announced his departure in a blistering farewell note to his colleagues, followed up on Wednesday with another searing attack, saying that the network was “doing a great, grave disservice to our country.”

The retired lieutenant colonel in the United States Army spoke on CNN in his first television interview since his departure. “With the rise of Donald Trump, Fox did become a destructive propaganda machine,” Mr. Peters said. “And I don’t do propaganda for anyone.”