In this May 25, 2017, file photo, Army Chief of Staff Gen. Mark Milley listens to a question while testifying on Capitol Hill in Washington, before a Senate Armed Services Committee hearing on the Army's fiscal 2018 budget. Credit: Andrew Harnik | AP

President Donald Trump announced Saturday that he would nominate Army Gen. Mark Milley as the next chairman of the Joint Chiefs of Staff, giving the president a critical opportunity to leave his imprint on the U.S. military by choosing its top officer.

In a pair of morning tweets, Trump said the transition date was to be determined. Milley is a four-star general who serves as Army chief of staff. Chairman Marine Gen. Joseph F. Dunford Jr. is scheduled to retire next fall.

“I am thankful to both of these incredible men for their service to our Country!” Trump tweeted Saturday.

Milley’s nomination will need to be confirmed by the Senate, which is unlikely to hold hearings for him or other just-announced presidential nominees before the new Senate convenes on Jan. 3.

The Joint Chiefs chairman oversees global military operations and is the president’s main adviser on military matters.

A veteran of the wars in Iraq and Afghanistan, Milley as Army chief championed a plan to create specialized units to train local forces in Afghanistan. He also worked to improve the Army’s readiness.

Some Pentagon officials expected the Joint Chiefs chairman to be selected from the Air Force, considering the informal rotation among the services. Dunford, a former Marine Corps commandant, succeeded Army Gen. Martin Dempsey as chairman in 2015.

Trump is scheduled to make an appearance Saturday at the annual Army-Navy football game in Philadelphia.