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.

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