Washington (CNN) When President Donald Trump's top national security advisers briefed Congress last week on intelligence used to justify killing Iranian general Qassem Soleimani, many lawmakers bristled at the defiant attitude of Trump's team — particularly his top general, chairman of the Joint Chiefs of Staff Mark Milley.

According to numerous people in the room, Milley was the most vocal defender of the intelligence. Multiple Hill Democrats said Milley came across as trying to match the bombast of the President in defending the strike.

One Democratic lawmaker told CNN they were turned off by Milley's pitch on behalf of the administration, particularly when he insisted that the intelligence was "exquisite." The lawmaker argued Milley's rhetoric was divorced from how intelligence is actually assessed.

"General Milley went out of his way to defend the intel behind the QS strike as so utterly compelling," a Democratic aide in the room told CNN. "It was embarrassing."

Not everyone reacted that way. Many Republicans and some Democrats came away impressed by Milley and defended him as an "honest broker" in the words of Virginia Democrat Sen. Tim Kaine.

Still, the briefing — and indeed the strike on Soleimani itself — marks a critical moment for the 61-year-old four-star general. At the peak of an illustrious military career, Milley's non-partisan reputation is being put to the test as he defends the most controversial foreign policy decision of Donald Trump's presidency.

In doing so, Milley has come dangerously close to wading into political realms that are usually out of bounds for the chairman of the Joint Chiefs, who is typically there to provide non-partisan military advice.

According to interviews with half a dozen former Pentagon and military officials, the consensus is that Milley is a brilliant and thoughtful military tactician, a scholarly thinker underneath a gritty exterior who has proven himself during numerous combat tours in Iraq and Afghanistan.

"The Chairman remains apolitical and focused solely on providing his best military advice to the President and the Secretary of Defense," Col. DeDe Halfhill, spokesperson for the Chairman of the Joint Chiefs of Staff, told CNN.

Yet some raised questions about Milley's ability to navigate the fraught politics that come with advising a mercurial president. Others on the national security team, including Secretary of Defense Mark Esper and National Security Adviser Robert O'Brien , have themselves struggled to defend in public Trump's shifting rationales for the Soleimani strike.

One retired military person who served under Milley in Afghanistan said that despite his spotless track record as a war-fighter, Milley may be "ill-prepared for the political realities of serving in the Trump administration."

"Trump has shown before that he is willing to take in current or former senior military leaders and exploit the credibility they provide, only to spit them out later with a tainted reputation," said this person. "It's reasonable to wonder whether Milley was sufficiently prepared for the pressures and expectations of this White House."

A general from central casting

Chairman of the Joint Chiefs of Staff Army General Mark Milley looks on during a welcome ceremony in Summerall Field, Joint Base Myer-Henderson, Virginia on September 30, 2019.

Milley likely fulfills one requirement Trump has for top advisers— that they look straight out of central casting. Barrel-chested with thick, dark eyebrows and a stern, square face, Milley more than fits the bill.

Multiple people who know him say he has a gruff, self-assured manner to match his tough visage. That also makes him the stylistic opposite of his predecessor as the chairman, the more reserved Marine Gen. Joseph Dunford.

"Dunford is very in control of himself," a former Defense official told CNN. "He would pull his glasses off and talk to you -- that's Dunford-speak for showing his frustration."

The former official said Dunford's successor brings a different energy to briefing the President -- less of a quiet voice of reason and more of a blunt, no-nonsense adviser.

"Milley is much more expressive, much more plain-spoken," said the official.

Retired Lt. Gen. Tom Spoehr, who served under Milley on the Army staff at the Pentagon, told CNN his former boss can be chatty and funny in private, and particularly likes to talk about professional sports in Boston, where he grew up.

That easy rapport was a big part of Milley's appeal to Trump. According to four people familiar, the President began interviewing the military chiefs of staff in the fall of 2018 to find a successor to Dunford. The Oval Office interviews were scheduled to last for 15 minutes. One leading candidate, Air Force Gen. David Goldfein, had a polite and formal conversation within the allotted time.

Milley, however, spent close to an hour with Trump, most of it in casual conversation, according to four sources familiar with the meeting. Even though then Defense Secretary James Mattis preferred Goldfein, Milley had the support of two key former Army officers close to Trump, Secretary of State Mike Pompeo and David Urban, a Trump campaign adviser and CNN contributor.

"A soldier's soldier"

Born and raised outside Boston, Milley went to Princeton, where he played on the hockey team and was in the Reserve Officers' Training Corps. He received his Army officer commission upon graduating, in 1980. Those who know him say it's been a point of pride for Milley that he began his career as a military officer in the ROTC program rather than the more traditional path of attending the United States Military Academy at West Point.

In his several operational deployments, including one tour in Iraq and three in Afghanistan, Milley gained a reputation as an out-of-the-box field tactician.

He is also the first chairman of the Joint Chiefs in nearly 20 years with a special-forces background, having graduated the Army's grueling Special Forces Qualification Course. That experience likely gives him unique insight into the ongoing conflicts in Syria, Iraq, Afghanistan and Africa, where special operations forces play a leading role.

"He's a soldier's soldier," said John Allen, the retired Marine four-star general who at one time was Milley's commanding officer in Afghanistan.

On the morning before he attended the Army-Navy football game in December 2018, Trump announced on Twitter he would pick Milley to succeed Dunford, though he was not formally nominated until April 2019.

At his confirmation hearing, Milley was asked about the possibility of a major military conflict with Iran.

"I don't think anyone is seriously considering [it]," Milley said at the time, noting that it would be disruptive to the administration's national defense strategy focusing on great powers like China and Russia.

In July, the Senate confirmed him 89 to 1 (with Oregon Democrat Jeff Merkley casting a lone nay vote), and Milley was sworn in as the 20th chairman on September 30.

Tumultuous from the beginning

Secretary of Defense Mark Esper and Chair of the Joint Chiefs of Staff Gen. Mark Milley arrive for briefing with members of the U.S. House of Representatives about the situation with Iran, at the U.S. Capitol on January 8, 2020 in Washington, DC.

From the moment he took the job, Milley has faced a series of conflicts and crises. Two weeks before he officially started, a suspected Iranian rocket attack on Saudi oil facilities prompted a redeployment of 3,000 US troops to the Arab country. A month later, Trump announced the surprise withdrawal of troops from Syria , leading to weeks of uncertainty about the future of the US presence in the region.

Milley agreed with Esper on the issue and offered a rationale for Trump's desire to remove troops, suggesting it was done to protect the lives of US soldiers to keep them from getting caught in the middle of skirmishes between the Kurds and Turks.

"He talks on behalf of soldiers ... it's a very personal approach," Republican Sen. Kevin Cramer told CNN, referring to how Milley discussed the Syria withdrawal with lawmakers. "I think it's helpful because it humanizes what can otherwise be sort of high level policy discussions."

In addition, Milley has had to deal with a couple military-related political blow-ups -- the controversial pardoning of Edward Gallagher , the Navy SEAL convicted of war crimes, and the publication in early December of a set of government papers revealing top US officials misled the American public about aspects of the War in Afghanistan.

When asked by a reporter after the release of the Afghanistan papers if he believed the country had been "throwing away American lives" in that war, Milley responded with a personal answer.

"I do not, absolutely do not," Milley said. "I could not look myself in the mirror. I couldn't answer myself at two or three in the morning when my eyes pop open and see the dead roll in front of my eyes."

"Tough cookie"

US President Donald Trump shakes hands with Joint Chiefs Chairman General Mark Milley after addressing the troops at Bagram Air Field during a surprise Thanksgiving day visit, on November 28, 2019 in Afghanistan.

Like Dunford, Milley is highly protective of his relationship with President Trump. Those who deal with him say he learned from Dunford that the way to retain his influence with the President is to ensure he never publicly reveals his conversations or advice to Trump.

Their friendly rapport was on display at Thanksgiving, when Trump traveled to Afghanistan for a surprise visit with US troops . Milley greeted Trump on the tarmac at Bagram Air Field, and later the two stood side by side serving Thanksgiving fare to hundreds of service members.

The two eventually sat down for a holiday meal together, but that was cut short by Milley, who, as the President jokingly recounted to the troops that day, pulled him into some photo ops.

"I had a gorgeous piece of turkey. And I was all set to go, and I had some of the mashed potatoes and I had a bite of mashed potatoes," Trump said. "And I never got to the turkey, because General Milley said, 'Come on over, sir. Let's take some pictures.'"

Trump also heaped praise on Milley, saying "It was my honor to appoint him. I had no doubt, from the day I met him — I met him very early on," Trump said from the flag-draped stage at Bagram. "And I said, 'I like that man. He's a tough cookie and he's smart.'"

Out in front

Milley's national profile has risen in the week since Soleimani's death. Along with Defense Secretary Esper, he presented to the President and was supportive of the plan to strike the Iranian general.

Since then, Milley has also taken on the role of public defender of the President's call. In front of reporters last week, Milley offered a full-throated defense of the decision to target Soleimani -- which he repeated two days later for lawmakers on Capitol Hill.

And after Iran's retaliatory missile strike on two Iraqi bases housing American troops, as some anonymous administration officials suggested Iran was not trying to target US soldiers, Milley stepped out to contradict them . The Iranians, he told reporters the next day, had intended to kill American troops and destroy vehicles and equipment.

"That's my own personal assessment," Milley said.

Not long after, Pentagon officials concurred with that view, and Vice President Mike Pence was echoing Milley in public interviews

The have also been snafus, most notably the surfacing of a letter last week announcing a US troop withdrawal from Iraq. Milley told reporters he immediately began calling commanders to determine the source of the letter. Milley eventually said the letter was a draft released by mistake , and not reflective of the Pentagon's plans.

Milley has also been forced to quiet concerns that the President would follow through on an apparent threat (via Twitter) to target 52 Iranian cultural sites if Iran killed American soldiers.

"We will follow the laws of armed conflict," Milley said Tuesday, a nod to laws that such targeting would constitute a war crime.

Milley has privately made clear that just like Mattis and Dunford he does not see his role as being the only so-called adult in the room to hold Trump back from his impulses. Rather, he perceived that his job is to deliver his best advice and the best options. Within that framework, he has made clear his willingness to disagree with the White House.

But at the moment, Milley is fighting the perception articulated by some lawmakers disappointed by his congressional briefings that he is too aligned with Trump.

"He's a loyal part of the Trump administration," Rep. Brad Sherman, a California Democrat on the Foreign Affairs Committee, told CNN Thursday. "He wasn't there to be a counterpoint to the decision-making process or lack thereof of this administration."

Friends like Spoehr recognize the risk Milley has taken.

"He is very aware that history will judge the joint chief when he comes out publicly to say the intel was compelling in this situation," Spoehr told CNN. "One day it'll become declassified, judged by history. He really put himself out there."