Hegseth is a decorated former National Guard infantry officer who served in combat, deploying to Iraq and Afghanistan, his website says, but he does not have the senior leadership experience typical of nominees to helm the world's most powerful military.

After joining Fox News in 2014 as a contributor, the 44-year-old now co-hosts Fox and Friends Weekend and is a host for streaming site Fox Nation. He has also authored multiple books.

Trump hailed Hegseth as "tough, smart and a true believer in America First" in a social media post announcing the nomination. "With Pete at the helm, America's enemies are on notice -- Our Military will be Great Again, and America will Never Back Down."

The president-elect also highlighted Hegseth's success as an author, saying his book "The War on Warriors" was a best-seller, and that it "reveals the leftwing betrayal of our Warriors, and how we must return our Military to meritocracy, lethality, accountability, and excellence."

Mark Cancian, a retired Marine colonel and senior advisor at the Center for Strategic and International Studies, said Hegseth's selection was unexpected.

"As someone said, this wasn't on anyone's bingo card, so it's a very surprising pick," he said. "Clearly Trump wants someone who is loyal to him" and who "does not have an independent political base."

- Potential 'civil-military crisis' -

Cancian said Hegseth "has an excellent record as a junior military officer" but "lacks high-level national security experience and experience leading a large institution," and could face significant opposition in Congress as a result.

Hegseth is a frequent critic of "woke" policies -- by companies, at universities, and in the military -- making it likely that he will scrap diversity programs if he is confirmed as Pentagon chief.

On a podcast aired earlier this month, Hegseth advocated for the firing of top US military officer General CQ Brown and said that any other senior officer who "was involved in any of the DEI woke shit has got to go."

Cancian said if the Trump administration stops at rolling back diversity programs it would resemble his first term in office, but that going further and taking a step such as firing Brown would "engender a civil-military crisis."

Hegseth said on the same podcast episode that he was opposed to women serving in combat, though not in the armed forces as a whole.

He also reportedly lobbied Trump in 2019 to intervene in three war crimes cases, leading to pardons in two murder cases and the reinstatement of the rank of a third service member who was convicted for posing for a photo with a body in Iraq.

Hegseth praised Trump's controversial involvement in the cases, saying on Fox News that the Republican "has emboldened the people out there making the impossible calls at impossible moments."

He is an Ivy Leaguer, graduating from both Princeton and Harvard, though his website says he sent his degree back to the latter institution, and he has criticized it on air for its allegedly left-leaning policies.

Hegseth lives with his wife and seven children in the southern US state of Tennessee.