Born in Detroit to a large family — more than 30 relatives attended his December swearing-in ceremony — Mr. Biegun was in high school in Pontiac, Mich., when a history teacher wrote the word “czar” on the chalkboard in the Cyrillic alphabet. He was immediately fascinated and went on to study Russian at the University of Michigan.

Mr. Biegun lived in Moscow in the early 1990s, when he worked for the International Republican Institute, which promotes democracy with some funding from the State Department and the United States Agency for International Development. But he mostly developed his national security credentials on Capitol Hill — first as a top Republican staff member on the Senate Foreign Relations Committee and later to Senator Bill Frist of Tennessee, then the majority leader — and at the White House as a top aide to Condoleezza Rice, who was the first national security adviser in the Bush administration.

He traveled to Russia as a vice president at Ford, negotiating new business ventures, but also took time off to briefly advise Sarah Palin, the Republican vice-presidential nominee in 2008. That position, according to colleagues, revealed his ability to maintain patience under pressure and to avoid a condescending tone — even when having to explain the most basic foreign policy axioms to his boss.

In his new job, Mr. Biegun will also remain the lead negotiator with North Korea — a dual role, he has said, that elevates “the priority on North Korea to the deputy secretary position, and I think that’s very important.”

But the diplomacy has fizzled since Mr. Trump and Mr. Kim abruptly left a summit meeting in Vietnam a year ago, unable to agree on a path for denuclearizing the Korean Peninsula. Critics say the Trump administration was too willing to keep the talks going — and the president too eager to meet with Mr. Kim — even as North Korea was busily building up its arsenal.

Mr. Biegun was not only trying to negotiate with the North Koreans, but he was also engaged in a behind-the-scenes fight with Mr. Trump’s national security adviser at the time, John R. Bolton, who believed Mr. Biegun was pursuing a useless mission.