US President Donald Trump on Saturday announced he was nominating Army chief of staff General Mark Milley as his next top military advisor -- a new slap in the face for Pentagon chief Jim Mattis.
Milley, a four-star general who has served in Iraq and Afghanistan, would replace General Joseph Dunford as the military's next chairman of the Joint Chiefs of Staff.
Dunford is scheduled to retire in October 2019 at the end of his second two-year term.
"I am thankful to both of these incredible men for their service to our Country!" Trump said on Twitter.
But in choosing the 60-year-old Milley, a graduate of Princeton University known for his combat command experience, Trump went against the wishes of Mattis, who reportedly favored Air Force General David Goldfein.
That choice would have been in keeping with a tradition of rotating the post among the services.
Dunford is a Marine Corps general. There has not been an Air Force officer in the chairman's post since 2005.
The Joint Chiefs of Staff is made up of the heads of the Air Force, Army, Marine Corps, Navy and National Guard.
Dunford quickly offered his congratulations to Milley.
"He has served with General Milley in peacetime and in combat and has the highest regard for his leadership," said Dunford's spokesman, Colonel Patrick Ryder.
Milley must be confirmed by the US Senate. He would take the position for a four-year term, after a new law was passed last year doubling the term length to better ensure continuity.
Trump said the transition date was "to be determined," in a sign that Dunford might go sooner rather than later, but Ryder noted: "All indications are that General Dunford will serve his full term."
Trump announced his picks for a new attorney general and a new UN ambassador on Friday and initially suggested he might make the military announcement at Saturday's annual Army-Navy football game in Philadelphia.
The game features cadets from the Naval Academy against their counterparts from the US Military Academy and is often attended by the president.
But he opted to announce the news on Twitter before heading to the game.
After meeting with both Milley and Goldfein, it seems that Trump preferred the army general, who was more outgoing than the cerebral Air Force chief, and laced his discussions with humor and historical references.
The Pentagon's reaction to the announcement was somewhat frosty.
"We are aware of the president's nomination and share his confidence for General Mark Milley," Pentagon spokesman, Lieutenant Colonel Mike Andrews, said in a brief statement.
"The Department of Defense remains fully focused on defending our nation."
Mattis was once a favorite of Trump, but the two men have had increasing difficulties in recent months.
They haven't seen eye to eye over the Iran nuclear deal, which Trump withdrew from over Mattis' objections; over the creation of a Space Force, which Mattis did not want; and over the crisis at the US-Mexico border, and the deployment of troops there.
Mattis and Dunford are expected to attend the Army-Navy game on Saturday along with Trump.