I strongly suspect that, either, the Russians set him up, although that would of course mean that he was dumber than dirt, or that he was indeed in cahoots with our old adversary, then served as a plant inside the Trump campaign.
Paul Manafort's dealings with Russia, via their guy in the Ukraine, is even more troubling. I suspect even more strongly that he was a plant as well. The guy who introduced Manafort to the Trump campaign (Thomas J. Barrack) is a very shady individual. He is on record, saying about Trump, "Not only is he crazy, he is stupid".
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Re: Michael Flynn
On February 13, 2017, Flynn resigned as National Security Advisor, following news reports about his communications with the Russian ambassador, Sergey Kislyak and additional reports that he had misled the Vice President [Pence] about them.[119]
Flynn's 24-day tenure as National Security Advisor was the shortest in the 63-year history of the office.[15]
Those communications he had with the Russian ambassador were subsequently leaked to the press.[120]
Commenting on Flynn's resignation, on February 14 White House Press Secretary Sean Spicer stated, "We got to a point not based on a legal issue, but based on a trust issue, where a level of trust between the President and General Flynn had eroded to the point where he felt he had to make a change ...
The issue here was that the President [Trump] got to the point where General Flynn's relationship – misleading the Vice President and others, or the possibility that he had forgotten critical details of this important conversation had created a critical mass and an unsustainable situation.
That's why the President decided to ask for his resignation, and he got it."[121]
https://en.wikipedia.org/wiki/Michael_Flynn
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Here, via C-Span transcripts, is the Wikipedia piece...
February 14, 2017
White House Daily Briefing
“The issue here was that the President got to the point where General Flynn’s relationship — misleading the Vice President and others, or the possibility that he had forgotten critical details of this important conversation had created a critical mass and an unsustainable situation.
That’s why the President decided to ask for his resignation, and he got it.
The irony of this entire situation is that the President has been incredibly tough on Russia.
He continues to raise the issue of Crimea, which the previous [Obama-Biden-Hillary] administration had allowed to be seized by Russia.”
https://www.c-span.org/video/?424005-1/sean-spicer-president-asked-michael-flynn-resign-eroded-trust
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"It was December 10 [2016], and Vladimir Putin was the guest of honor at RT’s 10th anniversary dinner, at Moscow’s Metropol Hotel in the shadow of the Kremlin. ...”
“Seated next to Simonyan at the dinner and just two seats away from Putin himself was perhaps the most intriguing example of how the Russians have gone about recruiting disaffected members of that establishment: a rugged-looking man in a tuxedo who less than 18 months earlier had been head of the Defense Intelligence Agency, the Pentagon’s powerful in-house equivalent of the CIA.
Michael Flynn, now a private citizen after a reportedly disgruntled retirement, was not there to gather intelligence.
His attendance at the RT [Russia Today] gala, before which he also gave a talk on world affairs, appeared to inaugurate a relationship with the network—presumably a paid one, though neither Flynn nor RT answered queries on the subject.
Flynn now makes semi-regular appearances on RT as an analyst, in which he often argues that the U.S. and Russia should be working more closely together on issues like fighting ISIL and ending Syria’s civil war.
“Russia has its own national security strategy, and we have to respect that,” he said in one recent appearance. “
And we have to try to figure out: How do we combine the United States’ national security strategy along with Russia’s national security strategy, despite all the challenges that we face...”
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Paul Manafort
Chairman of Donald Trump’s 2016 campaign
In February 2016, Manafort approached Donald Trump through a mutual friend, Thomas J. Barrack Jr.
He pointed out his experience advising presidential campaigns in the United States and around the world, described himself as an outsider not connected to the Washington establishment, and offered to work without salary.[49]
In March 2016, he joined Trump’s presidential campaign...”
In August 2016, Manafort’s connections to former Ukrainian President Viktor Yanukovych and his pro-Russian Party of Regions drew national attention in the US, where it was reported that Manafort may have received $12.7 million in off-the-books funds from the Party of Regions.[56]
On August 17, 2016, Donald Trump received his first security briefing.[57]
The same day, August 17, Trump shook up his campaign organization in a way that appeared to minimize Manafort’s role.
It was reported that members of Trump’s family, particularly Jared Kushner who had originally been a strong backer of Manafort, had become uneasy about his Russian connections and suspected that he had not been forthright about them.[58]
Manafort stated in an internal staff memorandum that he would “remain the campaign chairman and chief strategist, providing the big-picture, long-range campaign vision”.[59]
However, two days later, Trump announced his acceptance of Manafort’s resignation from the campaign after Steve Bannon and Kellyanne Conway took on senior leadership roles within that campaign.[60][61]
https://en.wikipedia.org/wiki/Paul_Manafort#Chairman_of_Donald_Trump’s_2016_campaign
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May 17, 2019...
After a quick (Wikipedia) search I found this on him...
Thomas J. Barrack Jr
After Trump became president, Barrack acted as a middleman between him and Arab princes.[37]
He denies a quote attributed to him in the 2018 book Fire and Fury that he said Trump was, “not only crazy” but “stupid”.[38]
In a 2017 Washington Post article, Barrack commented on Trump’s inflammatory rhetoric and proposals to ban immigrants from certain Muslim countries and put up a border wall with Mexico. “He’s better than this,” he said.[39]
Barrack was interviewed during the Special Counsel investigation into Russian interference in the 2016 election, in particular regarding Paul Manafort, Rick Gates, Konstantin Kilimnik, Cambridge Analytica, the Trump campaign, the Trump transition team, and the financing of the Trump inauguration.[40][41]
https://en.wikipedia.org/wiki/Thomas_J._Barrack_Jr.#Political_activity
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As you can see below, it's great to be a millionaire!, or billionaire?
Thomas J. Barrack