But in her transition role, Ms. Ricardel moved to block Mr. Mattis from selecting Anne Patterson, a career diplomat who had worked for Democratic and Republican presidents, to serve as under secretary of defense for policy, an influential post at the Pentagon. Administration officials have said the disagreements between Mr. Mattis and Ms. Ricardel held up senior appointments at the Defense Department for months.
After serving on the transition team, Ms. Ricardel moved to the Commerce Department. Now that she is at the White House, the friction between the Pentagon and the president’s national security advisers appears to have intensified.
The new dynamic between the president and his defense secretary is a dramatic turn from what was by all accounts a strong relationship between the two men when Mr. Trump first entered the White House. At the time, Mr. Trump spoke of Mr. Mattis in reverential and almost awe-struck terms, gleefully referred to him as “Mad Dog,” a nickname Mr. Mattis detests.
At a North Carolina rally where he announced Mr. Mattis as his pick for defense secretary, the president described the general as the living embodiment of the Marine Corps motto, “semper fidelis,” or “always faithful.”
“Mad Dog plays no games, right?” Mr. Trump bellowed, prompting cheers from the raucous crowd.
But as the months passed, Mr. Mattis found himself frequently treading a fine line on carrying out some of Mr. Trump’s orders, including moving slowly when the president tweeted an order to bar transgender troops from joining the military. The tweet, which came while Mr. Mattis was on vacation, caught the Pentagon brass off guard.
Now well into his second year as president, with Mr. Trump confident in his job as commander in chief and taking a more leading role on national security decisions, cracks in his relationship with Mr. Mattis are readily apparent. That includes sidelining Mr. Mattis over policy decisions and ignoring national security aides.
The recent publication of a book by Bob Woodward may have widened those cracks. In the book, Mr. Woodward writes that after a briefing on North Korea, Mr. Mattis told colleagues that Mr. Trump had the understanding of “fifth or sixth grader.” Mr. Mattis denied saying that.