WASHINGTON — President Trump said Saturday that he will nominate Army chief of staff Mark Milley as the next chairman of the Joint Chiefs of Staff, replacing Marine Gen. Joseph Dunford, with the date of the transition still to be determined.

Milley, who has served in Afghanistan, Iraq and Somalia, would become the country’s top military officer and senior military adviser to the president if confirmed by the Senate. He’d be responsible for helping to ensure that Trump’s policies in global hot spots are carried out.

One issue high on Milley’s agenda will be the 17-year war in Afghanistan, now the nation’s longest armed conflict. The Massachusetts native and Princeton University graduate knows that battlefield well: he’s served three tours in Afghanistan and was as the deputy commanding general for U.S. Forces-Afghanistan. He’s also served in Panama, Haiti and Bosnia-Herzegovina.

Milley would also be responsible for helping the military carry out the priorities in Trump’s National Defense Strategy, which calls for an emphasis on the potential for “great power” conflict with countries like Russia and China over a focus on counterterrorism.

Milley addressed the shift in priorities at an April hearing on Capitol Hill.

“Guerrillas, insurgents, terrorists are going to be around for a long time in various different forms,” he told the Senate Armed Services Committee on April 12. “We have to maintain the skill set. But, at the same time, we have to recapture our skills at combined arms maneuver warfare against near-peer competitors and great power competition.”

Milley, 60, would replace Dunford, who was first picked by President Barack Obama in 2015 and reappointed by Trump two years later. Dunford’s term is due to expire in 2019. Under a 2017 law, the term for the chairman of the Joint Chiefs will be increased to four years from two.

“It’s an unusual pick – Milley has extensive operational experience and excellent intellectual credentials, but not much” time working with the Navy, Marines and Air Force in joint operations, said Mark Cancian, a senior adviser for the International Security Program at the Center for Strategic and International Studies in Washington. “It’s certainly good news for the Army, which has struggled to make a place for itself in the new defense strategy.”

As Army chief of staff, Milley joined other senior military leaders in issuing what was interpreted as a subtle rebuke of Trump in August 2017, when the president blamed “both sides” for violence at rallies in Charlottesville, Va., when a white supremacist rammed his car into a crowd, killing a woman.

“The Army doesn’t tolerate racism, extremism, or hatred in our ranks,” Milley wrote on Twitter. “It’s against our Values and everything we’ve stood for since 1775.”