Washington(CNN) As President Donald Trump grapples with a number of huge foreign policy challenges over the coming months, he is going to have to make some key decisions on the generals who advise him and lead the US military.

Behind the scenes, the White House and Pentagon are looking for a new slate of senior officers to fill several key jobs with several senior military figures set to retire.

Defense Secretary James Mattis is playing a central role in making recommendations for nominations that will have to be confirmed by the Senate. No one knows how long Mattis will stay in his job or the names he is recommending. But the new crop of senior commanders could help shape military operations and policy for some years to come.

A key vacancy will be filled in early 2019, when the highly respected four-star Air Force Gen. Paul Selva, currently the vice chairman of the Joint Chiefs of Staff is scheduled to retire. As vice chairman, Selva holds significant responsibilities overseeing highly classified nuclear weapons procedures, as well as military oversight of billions of dollars in budgeting and new weapons acquisitions.

Some defense officials believe Army four-star Gen. Vincent Brooks, who heads the US military command in South Korea, could be a leading candidate to replace Selva.

Brooks is scheduled for rotation out of South Korea in the coming weeks, but in a potentially crucial signal, his retirement has not been announced and some believe he is being considered for a higher-level job. Brooks would provide President Trump with valuable military expertise on North Korea as denuclearization talks are ramping up.

Trump also has to pick a new chairman of the joint chiefs, the top military adviser to the president later in 2019. The current Chairman, Gen. Joseph Dunford, is scheduled to complete his second two-year term in the fall of 2019.

The jobs of chairman and vice chairman must be filled by personnel from different services, so if Brooks is the new number two, President Trump will have to nominate a new chairman from a different service. That would shut out Gen. Mark Milley, the Army Chief of Staff and combat veteran who Trump has interacted with in the past, from the top job.

Defense officials emphasize they don't know who the President will pick as the next chairman, but there are suggestions that Gen. David Goldfein, the current Chief of Staff of the Air Force, could be a frontrunner for nomination to the top job. It would put in place a top military official to push through Congress the President's idea for a new Space Force. Goldfein was shot down over Serbia in his F-16 and rescued from behind enemy lines in 1999.

Another contender for the top job would be Air Force Gen. John Hyten, currently head of the Strategic Command who has crucial expertise on nuclear weapons. An Air Force officer has not served in the top job since 2005, so there is a sense in the senior ranks that it's the Air Force's turn to fill the role.

If neither of them are selected, it could open up the vice chairman job to Milley.

All of this comes as other key senior commanders overseeing key geographic areas are scheduled for retirement. Marine Corps Lt. Gen. Kenneth McKenzie is slated to take over at US Central Command from Gen. Joseph Votel, overseeing operations in the Middle East including Iraq, Syria and Afghanistan—all combat operations that President Trump has criticized.

Army Lt. Gen. Richard Clarke has been selected to lead US Special Operations Command, replacing Gen. Raymond Thomas and overseeing thousands of special forces deployed around the world.

Air Force Gen. Tod Wolters is now considered the top candidate to lead the European command and become NATO's Supreme Allied Commander Europe, essentially the President's point man on military interactions with Russia. Wolters currently heads US Air Force operations in Europe. He would replace Gen. Curtis Scaparrotti, who is also scheduled to retire.

CORRECTION: This story has been updated to correct the spelling of Wolters' name.