In his three weeks on the job, Defense Secretary Pete Hegseth has removed portraits of his predecessors from the Pentagon, banned Black History Month celebrations, restricted access to learning materials in military schools that touch on views he doesn’t like and attacked the renaming of bases that had honored Confederate generals.

On his first trip abroad in the role, he said in Brussels on Wednesday that it was unrealistic for Ukraine to regain all the territory seized by Russia. And he signaled to Europeans that they should look to themselves rather than the United States to ensure their security.

All of the new cabinet secretaries have embraced their role as promoters of President Trump’s slashing brand of politics. Few, though, have done so with such fervor as Mr. Hegseth, a former National Guard infantryman and Fox News host who shares the president’s view that allies have taken advantage of U.S. largess for too long but who brings a special intensity to perceived enemies at home.

During his first meeting with Pentagon employees at a town hall event on Friday, Mr. Hegseth spent much of his opening remarks defending his efforts to dismantle diversity and inclusion policies. He repeated his talking points about “lethality” and bringing “warfighting” back to the military. He briefly spoke about “Communist China,” but said nothing about Iran, North Korea or Russia.

Deterrence, he said, “starts with our own southern border.”

“It starts with the defense of our homeland,” he continued.

It was a view he reiterated on Tuesday, after meeting with U.S. troops and commanders in Stuttgart, Germany. Asked at a news conference if China was the biggest threat to the United States, he replied, “Right now, the biggest threat was securing our own border, which we are addressing rapidly.”