Gen. Mark Milley is expected to become the next chairman of the Joint Chiefs of Staff. (Andrew Harnik/AP)

President Trump is expected to choose the head of the Army to become the next chairman of the Joint Chiefs of Staff, tapping a voluble and unconventional combat veteran to become the United States’ top military officer, individuals familiar with White House plans said Friday.

In a move that reflects his penchant for showmanship, the president plans to announce his nomination of Gen. Mark Milley at Saturday’s annual Army-Navy football game, ending months of speculation about who will replace the current chairman, Gen. Joseph F. Dunford Jr., who is due to step down next fall.

According to the individuals, who spoke on the condition of anonymity to discuss a decision that has not been made public, Trump considered two senior officers, Milley and the Air Force chief of staff, Gen. David Goldfein, whom Defense Secretary Jim Mattis preferred.

The selection of a chairman, who oversees global operations and serves as the president’s chief adviser on military-related matters, represents an important opportunity for Trump to make his mark on the U.S. military.

If confirmed by the Senate, Milley would bring to the job a distinguished record as a commander in the counterinsurgency wars of the past two decades. A graduate of Princeton University, Milley served as a Green Beret and later commanded troops in Iraq and Afghanistan.

As the Army chief, Milley championed a proposal to create specialized units to train local forces in Afghanistan while also seeking to improve the Army’s readiness as the Pentagon reorients toward challenges from Russia and China.

Trump’s selection of Milley defies an expectation among many Pentagon officials that the next chairman would be selected from the Air Force, in keeping with an unofficial alternation between the services. Dunford, a Marine, replaced an Army chairman in 2015; previously, a Navy admiral served in the job.

It’s not clear why Mattis preferred Goldfein, another widely respected, cerebral officer. Some current and former officials cited the president’s choice as a sign of the Pentagon chief’s diminished influence with the White House.

“It’s a pretty big decision to go against Mattis,” said one former top defense official who spoke on the condition of anonymity.

Current and former officials said Mattis has a good relationship with Milley. When the defense secretary wanted to rethink the war in Afghanistan, he took the unusual step of going to Milley to brainstorm, even though as Army chief of staff he did not have direct oversight of the war, said a former senior defense official.

Mattis was frustrated by the Army’s inability to slim down big headquarters in Afghanistan and push more soldiers into an active role supporting Afghan troops in the field. Milley was seen as the kind of officer who could produce a more unorthodox approach, officials said.

The result of that effort was the Security Force Assistance Brigade, now seeking to help Afghan troops reverse a prolonged comeback by the Taliban.

Milley also dedicated time to making sure the U.S. military understood the risks and was prepared for a possible conflict with North Korea. Inside the Pentagon, he warned that any conflict with Pyongyang would lead to massive loss of life and catastrophic damage.

At the same time, he pushed to improve the readiness of the joint force to repel or carry out an attack on North Korea, if ordered, said a senior Army official. He also pushed military planners to think more broadly of how such a war might be fought, officials said. Most of the Army’s plans for a war on the peninsula were predicated on North Korean aggression rather than American offensive action to blunt a growing North Korean threat.

Milley is poised to ascend to the top uniformed position at a challenging moment for the military’s all-volunteer force. The military must contend with a politically polarized country, a shrinking pool of recruits and a mandate to shed the extremist focus of the post-9/11 era and focus on major powers such as Russia and China.

One of Milley’s biggest challenges may be handling an impulsive commander in chief, whose rhetoric and actions, including his decision to deploy active-duty forces to the southern border ahead of the midterm elections, have threatened to politicize the force.

Many in the Pentagon privately questioned the need for the deployment and worried the troops were being used to a political end.

Dunford and Mattis have sought to navigate those challenges in part by attempting to shield the military. Both have declined to speak publicly about their views in areas where Trump has stirred controversy. Dunford has emphasized his duty to execute legally issued orders, no matter his opinions.

His relationship with Trump has been more distant than his predecessor, Army Gen. Martin Dempsey, had with President Barack Obama.

It’s not clear how Milley will navigate potential pitfalls. Occasionally, his willingness to speak out has appeared to put him at odds with his commander in chief.

After Trump was criticized for appearing to condone some of the white-nationalist protesters in Charlottesville in 2017, Milley tweeted the Army “doesn’t tolerate racism, extremism, or hatred in our ranks,” seemingly distancing himself from Trump’s comments.

Milley is likely to elevate the profile of the chairman. Dunford has shied away from taking a public role as the military’s top officer, matching his circumspect personality.

By contrast, Milley is an ebullient personality and natural storyteller who could follow the path of Adm. Michael Mullen, a former chairman in the Obama administration. Concerned about the growing gulf between the military and the society it was sworn to defend, Mullen not only spoke on news shows, but also made an appearance on the Late Show with David Letterman.

Milley, who attended prep school in Massachusetts and then Princeton, has a somewhat unusual background for a top military officer. He played hockey with and remains is a longtime friend of TV producer and writer David E. Kelley.