New acting Pentagon chief Patrick M. Shanahan made clear Wednesday that the Trump White House will have a long-sought ally inside the Defense Department in the wake of former Defense Secretary James Mattis’ abrupt departure, while telling aides a primary focus of his tenure will be the threat posed by China.

Appearing at Mr. Trump’s side during Wednesday’s Cabinet meeting at the White House on Wednesday, Mr. Shanahan said the Army Corps of Engineers was “dialed in” to help construct additional parts of Mr. Trump’s Mexico border wall and that the Pentagon was exploring how to strengthen its support of the Department of Homeland Security’s drive to curb illegal immigration.

“The threat is real, the risks are real,” the former top Boeing executive said in the publicly televised portion of the meeting.

But Mr. Shanahan sat quietly later as Mr. Trump belittled key U.S. allies as freeloaders and laid into Mr. Mattis, who officially resigned Dec. 31, during a subsequent Q&A session with reporters. Mr. Trump claimed he had “essentially” fired the former four-star Marine Corps general and criticized in particular Mr. Mattis’ handling of the war in Afghanistan.

“As you know, President Obama fired him, and essentially so did I,” Mr. Trump told reporters of Mr. Mattis’ departure. “What’s he done for me? How had he done in Afghanistan? Not too good,” he said.

After a string of policy clashes over the past nearly two years, Mr. Mattis resigned abruptly in the wake of Mr. Trump’s order late last month to withdraw all 2,000 U.S. forces currently deployed to Syria, and slash the 14,000 U.S. forces in Afghanistan by half in the coming year, citing in part the strain Mr. Trump’s policies have placed on America’s allies.

Mr. Mattis had intended to stay on until February, but Mr. Trump announced he would be leaving his post in the new year, elevating Mr. Shanahan, who had no experience at the Pentagon prior to his appointment last year, to the top job.

Already some other personnel moves are in the works: Mr. Shanahan named Pentagon Comptroller David L. Norquist to serve as deputy secretary on an interim basis. Mr. Norquist will continue to serve in his role as comptroller, officials said.

As the department’s top accountant, he helped shepherd the Trump administration’s fiscal year 2019 defense spending package into law, and played a key role in drafting the Pentagon’s budget blueprint for fiscal year 2020.

Meeting with top aides at the Pentagon on Wednesday morning, Mr. Shanahan signaled the Pentagon under his watch will be directing more of its focus to the threat posed by China, a national security imperative championed by the Trump administration.

“While we’re focused on ongoing operations, acting Secretary Shanahan told the team to remember China, China, China,” a defense official told reporters Wednesday at the Pentagon, ahead of Mr. Shanahan’s appearance at the White House.

Ongoing friction between Washington and Beijing over territorial claims in the South China Sea and elsewhere, coupled with the ongoing trade war, has exacerbated already frayed Sino-U.S. relations.

Chinese President Xi Jinping further inflamed those tensions Wednesday, stating Beijing would be willing to use “force” to absorb Taiwan under the rule of mainland China. Beijing has long dismissed Taipei’s claims of independence, while the U.S. has maintained a strong military alliance with the island nation 110 miles off the Chinese coastline.

For his part, Mr. Shanahan appeared to think the day well, tweeting Wednesday evening: “Had a great day today — participated in a Cabinet meeting at the @WhiteHouse, met w/ DOD senior leaders & engaged w/ my policy team.”

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