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As one fellow officer put it, referring to Trump's inner circle of aides and speaking on condition of anonymity, the Trump White House "has its own Republican Guard, which may be harder for him to deal with than the Iraqis were." The Iraqi Republican Guard was the elite military force of ousted dictator Saddam Hussein.

McMaster's fame grew after his 1997 book "Dereliction of Duty" criticized the country's military and political leadership for poor leadership during the Vietnam War.

Trump's pick was praised by one of the president's strongest backers in the U.S. Congress, Republican Senator Tom Cotton, who called McMaster "one of the finest combat leaders of our generation and also a great strategic mind. He is a true warrior scholar, and I'm confident he will serve both the president and the country well."

'CRITICISM AND FEEDBACK'

In a July 14, 2014, interview with the Columbus Ledger-Enquirer in Columbus, Georgia, where Fort Benning is located, McMaster, then the base commander, said: "Some people have a misunderstanding about the Army.

"Some people think, hey, you’re in the military and everything is super-hierarchical and you’re in an environment that is intolerable of criticism and people don’t want frank assessments.

"I think the opposite is the case ... And the commanders that I’ve worked for, they want frank assessments, they want criticism and feedback."

That attitude was not always shared by his superiors, and it led to his being passed over for promotion to brigadier general twice, in 2006 and 2007.

On McMaster's third and last try, General David Petraeus – who took himself off the list last week for Trump's national security adviser – returned from Iraq to head the promotion board that finally gave McMaster his first general's star.

Then a colonel, McMaster was commander of the 3rd Armored Cavalry Regiment that in the spring of 2005 captured, held and began to stabilize Tal Afar on the Iraqi-Syrian border.

The city was held by Sunni extremists, a crossing point between Syria and Iraq for jihadists who started as al-Qaeda in Mesopotamia under Abu Musab al-Zarqawi and morphed into Islamic State after he was killed.

McMaster's preparation of the regiment is legendary: He trained his soldiers in Iraqi culture, the differences among Sunnis, Shiites and Turkomen, and had them read books on the history of the region and counterinsurgency strategy.

It was a sharp change from the "kill and capture" tactics the United States had used in Iraq since the invasion in March 2003, and to which the Obama administration returned in Afghanistan, Iraq and Syria.