When Pete Hegseth was an earnest, young Army lieutenant in Iraq in 2005, he was cleareyed on how he viewed crimes committed by soldiers in war.

Soldiers in his own infantry company in Iraq in 2006 had shot civilians, executed prisoners and tried to cover up the crimes.

“Those are a no-brainer,” he told an audience at the University of Virginia after his deployment. He called the acts of those soldiers, who served in a sister platoon in his company, “atrocities” and added: “Of course that’s wrong. No one is here to defend that.”

By the end of his Army career, though, he was repeatedly doing exactly that.

As a presenter on Fox News, he portrayed other troops charged with war crimes as “heroes.” The military prosecuting them was, he said, “throwing warriors under the bus.” The once circumspect officer glossed over crucial details, told his TV audience that troops were just “doing the job they were hired to do” and pushed relentlessly for President Donald J. Trump to intervene.

It was a stark shift for the man President-elect Trump picked this month to lead the Defense Department. Soldiers who served with Mr. Hegseth say the change was driven in part by a string of military deployments — once to Guantánamo Bay, once to Iraq and once to Afghanistan — that each taught him a new lesson in military dysfunction.

The experience transformed him from a neoconservative believer in U.S. military might into an outsider so distrustful of the national security establishment that he repeatedly sided with convicted murderers over Pentagon leadership.