MICHAEL LEDEEN: Make No Mistake: The Mueller Investigation Was All About Flynn.
By the time he was made head of DIA, Flynn had a real problem with the intelligence community, first because he had marginalized them, and for another reason: Flynn was determined to do a full-scale analysis of the (many) secret missions that had not been carried out over the years, and he wanted an accounting of the considerable funds allocated for them.
So there were many high-ranking intelligence officials who were out to get Flynn. You can see them at work long before there was a hint of Russiagate, when the target was not yet Donald Trump. But then things got worse for the IC, when Flynn was named to head DIA. By then, the FBI was fully engaged in the anti-Flynn campaign, paying people like Stefan Halper to surveil Flynn’s behavior in Great Britain. This produced the fanciful accusation (impossible, for anyone who knew the general) that Flynn had flirted with a good-looking Russian historian. This may have been the start of the “collusion” allegations.
Needless to say, read the whole thing.