He was acting as an unregistered foreign agent on behalf of Turkey, penning op-eds advocating foreign policy initiatives that were strongly favored by Turkey.

He received money from a Russian state owned organization to speak at an event.

He dined at the same table with Vladimir Putin (and Jill Stein, and several of Putin's close aides) at an event celebrating the 10th anniversary of Russian propaganda news outlet RT - an event at which Flynn was paid $45k even though (or perhaps because) he was then advising then-candidate Trump on his Presidential campaign. Three of the Putin aides at the table were then under US sanctions relating to Russia's annexation of Crimea.

He had phone conversations with Sergei Kislyak prior to his appointment and prior to Trump's inauguration. The conversations were monitored by intel agencies as part of their routine monitoring of foreign official communications. One of them happened on the same day the US announced it was expelling around 3 dozen Russian diplomats as a consequence of the intel community's determination that Russia interfered in the 2016 election,and shortly thereafter, rather than engage in tit for tat, Putin announced he was not expelling US diplomats but even invited them and their families to a New Years celebration.

Flynn then lied about the content of those conversations to VP Pence and then-incoming Press Secretary Sean Spicer, stating in response to media reports that they were about the sanctions (basically him telling Kislyak to hold off on anything until Trump was inaugurated) that they were not. Pence and Spicer then repeated the lie (likely unknowningly).

The FBI interviewed Flynn about the conversations and he more or less repeated the lie that hey had nothing to do with sanctions and were just normal establishment of communications lines between incoming national security officials and their Russian counterparts.

He pled guilty (and reaffirmed his guilt under oath twice) to lying to the FBI.

These are facts. Not really controverted anywhere. Not even Barr's motion to withdraw the prosecution challenges these facts (other than to slyly imply Flynn might not have been lying, but if he did it wasn't "material" anyway).