On Feb. 13, 2017, Flynn resigned after the White House learned he had lied to Vice President Mike Pence about those conversations. He became the first in a string of high-profile departures from the Trump administration as the Mueller probe and other scandals have swirled around the White House.
It was a staggering downfall for a former Army lieutenant general who helped lead efforts to strengthen military intelligence operations in Iraq and Afghanistan, and later led the Pentagon's Defense Intelligence Agency.
After pleading guilty, Flynn cooperated extensively with Mueller's team, court papers show.
Citing that assistance, prosecutors urged Sullivan last year to spare Flynn prison time. Under federal sentencing guidelines, which judges use to help mete out appropriate punishment, he faces no more than six months in jail.
In June, two months after Mueller had issued his final report, Flynn abruptly changed tactics.
He fired his lawyers who had negotiated the guilty plea and replaced them with a team led by Sidney Powell, a pro-Trump attorney who frequently appears on conservative television and radio shows.
In seeking to toss the case and obtain more evidence, Powell and her colleagues in recent months have accused the FBI of plotting to trap Flynn. Agents orchestrated an "ambush-interview," Powell wrote, and tricked her client "into making statements they could allege as false."
Flynn's new lawyers have also savaged the credibility of a former FBI agent, Peter Strzok, who had helped question Flynn. Strzok was fired in August 2018 after Justice Department officials uncovered private anti-Trump texts he had exchanged with an FBI employee with whom he was having an affair.
Arguing that prosecutors withheld helpful evidence, the defense lawyers have sought to link Flynn's plight to a number of conspiracies involving the Russia investigation, though none appear to directly involve Flynn's lies to the FBI or Pence.
Most recently, Flynn's new team has sought access to cellphones purportedly belonging to Joseph Mifsud, a Maltese academic who told a Trump campaign adviser that Russia had "dirt" on Democratic presidential candidate Hillary Clinton in the form of "thousands of emails." That tip ultimately led to the FBI investigation of Russian meddling in the 2016 race.