(18.10.2018) The three people said Mr. Kelly and Mr. Bolton were sparring over an issue that has frequently angered the president and has often prompted him to lash out at Kirstjen Nielsen, who succeeded Mr. Kelly as the secretary of homeland security after serving as his deputy chief of staff in the White House. Mr. Bolton was siding with the president, who has angrily blamed Ms. Nielsen for failing to stanch the flow of migrants across the border, while Mr. Kelly, who is fiercely protective of his protégé, defended her, the people said.