Michael Flynn's former colleagues describe him as volatile and 'unbelievably arrogant'
But then, in 2014, he got fired as the head of the Defense Intelligence Agency. US officials cited clashes over his leadership style, but he claims it was because he took a stand about "radical Islam."
Eventually, he wrote a book about how to "win the global war against radical Islam" and became one of Trump's most ardent supporters, leading The Washington Post to ask, "What the heck has happened to Michael Flynn?"
Some in the military and intelligence communities were shocked to see Flynn encouraging chants of "Lock her up!" at the Republican National Convention, referring to Trump's opponent, Democratic presidential nominee Hillary Clinton. And later in the campaign season, reports surfaced of Flynn clashing with officials who conducted a national security briefing for Trump.
Flynn's most recent controversy involved his son, who promoted the "pizzagate" conspiracy theory on Twitter, which baselessly alleged through fake news stories that a pedophilia ring involving high-level Democratic Party officials was operating out of a Washington pizzeria. Flynn himself has promoted fake news stories on his Twitter feed.
Gen. Barry McCaffrey told MSNBC recently that some of Flynn's tweets "border on demented."
Daniel W. Drezner, a professor of international politics at the Fletcher School of Law and Diplomacy at Tufts University, concluded in his column for the Post that Flynn "should be kept as far away from power as humanly possible."
Some of Flynn's former coworkers painted a picture of the lieutenant general that seems to support that concern.
Sarah Chayes, who is now at the Carnegie Endowment for International Peace, was a special adviser to Gen. Stanley McChrsytal, who worked very closely with Flynn, in Afghanistan in 2009. She told Business Insider she shared an office with Flynn for more than a year during this time.
She described a "14-year-old disruptive kid" who often contradicted himself.
"I would put this like - imagine if the kid, the sort of transgressive, fun, energetic, arrogant boy in the back of the class, that that's like who you're working with," Chayes said. "It was very high-energy, it is fun, he's fun to be around, but unbelievably arrogant."
Chayes said that when McChrystal's team, including Flynn, who was a military intelligence officer at the time, arrived at a NATO headquarters in Afghanistan, they "blasted in basically saying everything was all screwed up."
They said "they were going to, not only they could fix it, they would fix it, but it was like … the implication was very insulting to the entire command and that obviously felt like something that was problematic," she said.
"It was not only the individual officers being insulted, but countries we're in an alliance with. ... They had put lots of people and efforts and energy and materiel into this fight," Chayes continued.
Chayes described a person who spoke in constant contradictions.
"You listen to him closely for six or seven minutes and he will have contradicted himself two or three times," she said.
She also described him as a rule-breaker, which fits with what he wrote in his book about being a "maverick."
"There was a sense in that headquarters and with him ... kind of like the rules didn't apply to him," Chayes said.
But she also described him as a good communicator who encouraged open dialogue.
One colleague noted to the New Yorker in November that Flynn would encourage junior officers to challenge him in briefings, despite his high rank. The unnamed colleague said junior officers "just loved him."
Another source told the New Yorker that Flynn often wrote "This is bullshit!" in the margins of classified papers he passed up the chain.
Someone who worked with both McChrystal and Flynn said McChrystal encouraged Flynn to control his outbursts and listen to colleagues who challenged the unsubstantiated theories he promoted.
Many former colleagues who have talked to the press (often anonymously) since Flynn was named Trump's national security adviser said that while he is tactically brilliant, he struggles with broad strategy.
"He made a lot of changes," one source told the New Yorker of Flynn's time at the DIA. "Not in a strategic way - A to Z - but back and forth."
Retired Navy Adm. James Stavridis, who is currently the dean of the Fletcher School, made a similar assessment.
In an email to Business Insider, he described Flynn as "a superb tactical commander both in Afghanistan and Iraq, with a very hard-edged view of Islam and a direct, straight-ahead style of leadership."
"As national security adviser, he will easily do the 'adviser' part of the role, which is in his wheelhouse," Stavridis said. "But he will have to develop the ability bring opposing views from each of the cabinet officials into synthesis for the president. It is a very tough job."
Rep. Devin Nunes, who is chairman of the House Intelligence Committee and a member of Trump's transition team, said Flynn is up for the job.
Nunes said he has known Flynn for a long time and has been briefed by him dozens of times. He praised Flynn as "one of the best and most knowledgeable generals" he's seen during his career in Congress.
"He's perfect because the president-elect trusts him and many of us in Congress trust him," Nunes told Business Insider.
He pushed back on the unflattering portraits of Flynn that have appeared in the press.
"You don't have a 30-year career, develop one of the most successful military operations in the history of warfare and all of the organizations he's run, all the promotions he's received … and then have a problem running an agency," he said. "You know, it didn't add up at the time."
Nunes pointed to Flynn's experience fighting Al Qaeda in the Middle East.
"He was the intelligence officer in charge of the machine led by Stan McChrystal that tracked down and killed Al Qaeda, so we know he knows how to do it," he said. "And he's one of the few who would know how to do it who has the experience."