WASHINGTON — The departure of Defense Secretary Jim Mattis in a week will thrust President Donald Trump’s largely unknown choice for acting Pentagon chief into the military hot seat and shift attention to the search for a permanent replacement who will probably face sharp Capitol Hill questioning about the administration’s murky foreign policy.

Trump tweeted Sunday that Deputy Defense Secretary Patrick Shanahan will take over as acting head of the department on Jan. 1, elevating a former Boeing Co. senior executive with little experience in international affairs. Shanahan worked for Boeing for more than three decades and was a senior vice president when he became Pentagon deputy in July 2017. And his time on the job has been mainly focused on the business side of the department and its budget in excess of $700 billion.

A White House official said that in the new year Trump wants to focus on streamlining purchases at the Pentagon, an issue on which Shanahan has already been working. The official asked not to be identified publicly discussing personnel matters.

But there are looming policy questions about the wars in Syria and Afghanistan, including critical decisions about how the Pentagon will carry out Trump’s order last week to pull all 2,000 U.S. troops out of Syria, and withdraw up to half of the 14,000 American forces in Afghanistan.

Both decisions trigger massive logistical challenges to get the troops and equipment out of both warzones safely, without further inflaming U.S. allies or boosting the aspirations of the enemy.

A key unanswered question is what the administration will do for the U.S.-backed Syrian Democratic Forces, which have gotten weapons and training to fight Islamic State insurgents. The SDF will face almost certain attacks from Turkey, the Syrian government and IS once the U.S. leaves, and officials don’t know if the Syrian rebels will have to return the weapons to the U.S.

Shanahan hasn’t been to either war zone, and officials say he’s likely to depend heavily on Marine Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff. Dunford is scheduled to leave the job and retire Sept. 30, and Trump has already said he is nominating Gen. Mark Milley, the Army chief, as the next chairman.

Dunford, who was in Afghanistan on Monday with a USO show, has been telling troops that so far the mission in Afghanistan has not changed, said his spokesman, Air Force Col. Pat Ryder. Ryder said Dunford “told them that they’re American soldiers, they have a mission to do, and to just get after it.”