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(CNN) —  

President Donald Trump on Saturday nominated Army Chief of Staff Gen. Mark Milley to be the next chairman of the Joint Chiefs of Staff.

“I am pleased to announce my nomination of four-star General Mark Milley, Chief of Staff of the United States Army – as the Chairman of the Joint Chiefs of Staff, replacing General Joe Dunford, who will be retiring,” Trump posted on Twitter Saturday morning. “I am thankful to both of these incredible men for their service to our Country!”

Trump said the date of transition is still to be determined.

Marine Corps Gen. Joseph Dunford, who took on the role in 2015, is expected to serve out his term, which expires later next year.

The chairman is the highest-ranking military officer in the country and serves as the principal military adviser to the President. It is a role that requires Senate confirmation.

Milley interviewed with Trump for the position on November 14 for more than an hour, a source told CNN.

“He’s a great gentleman, he’s a great patriot, he’s a great soldier. And I look forward to that,” Trump told reporters later Saturday, praising Milley after announcing his nomination.

Joint Chiefs spokesman Col. Patrick Ryder told CNN that Dunford congratulates Milley and “has the highest regard for his leadership.”

Pentagon spokesman Lt. Col. Mike Andrews said, “We are aware of the President’s nomination and share his confidence” in Milley to be the Joint Chiefs chairman.

In his more than 35 years in the military, Milley has commanded units with the 10th Mountain Division and the 101st Airborne and served multiple combat tours in Iraq and Afghanistan.

He was the head of the US Army Forces Command at Fort Bragg, North Carolina, and was assigned to review the investigation of former Army Sgt. Bowe Bergdahl, who was held captive by the Taliban for five years after leaving his post in Afghanistan in 2009. Milley made the decision to charge Bergdahl in 2015 with desertion and misbehavior before the enemy, for which Bergdahl ultimately received a dishonorable discharge but avoided prison time.

Milley was nominated to the position of Army chief of staff by President Barack Obama and confirmed by the Senate in August 2015.

After becoming Army chief of staff, Milley helped oversee the Army’s transition away from large-scale counterinsurgency campaigns in places like Iraq and Afghanistan, focusing instead on challenges from Russia and China.

In a statement, South Carolina Republican Sen. Lindsey Graham called Milley “a great choice.”

“He’s a battle-tested commander and Pentagon reformer who will be a worthy successor to General Dunford,” said Graham, who sits on the Armed Services Committee.

A native of Massachusetts, Milley holds degrees from Princeton University and Columbia University.

If confirmed, Milley will be the first chairman of the Joint Chiefs of Staff to serve a single four-year term, a new policy made law in 2017.