"Pete is tough, smart and a true believer in America First," Trump said in a statement. "With Pete at the helm, America's enemies are on notice - Our Military will be Great Again, and America will Never Back Down."

Hegseth is an Army National Guard veteran and according to his website served in Afghanistan, Iraq, and Guantanamo Bay, Cuba.

Hegseth has said he left the Army in 2021 after being deemed an extremist by an Army that didn't want him anymore.

"The feeling was mutual -- I didn't want this Army anymore either," Hegseth said in his book "The War on Warriors: Behind the Betrayal of the Men Who Keep Us Free."

There is already anxiety in the Pentagon that Trump aims to root out military officers and career civil servants he perceives to be disloyal.

Culture war issues could be one trigger for firings. Trump was asked by Fox News in June whether he would fire generals described as "woke," a term for those focused on racial and social justice but which is used by conservatives to disparage progressive policies.

"At a basic level, do we really want only the woke 'diverse' recruits that the Biden administration is curating to be the ones with the guns and the guidons?" Hegseth wrote in "The War on Warriors," which was published in June.

"We want those diverse recruits -- pumped full of vaccines and even more poisonous ideologies -- to be sharing a basic training bunk with sane Americans," he said.

Trump's former U.S. generals and defense secretaries are among his fiercest critics, with some declaring him unfit for office. Angered, Trump has suggested that his former chairman of the Joint Chiefs of Staff, Mark Milley, could be executed for treason.