For some unfathomable treason, Flynn spoke unguardedly with Kislyak in several calls on December 29, the same day that outgoing President Barack Obama announced measures designed to punish Russia for interfering in the election.
The calls, we now know, were monitored by US intelligence agencies.
On January 12, the first hint that trouble was brewing for Flynn appeared in the Washington Post when an article disclosed that he had been in contact with Kislyak.
A US law specifically bars unauthorised citizens from contact with foreign powers in a way that could influence disputes with America, just as US law comes down hard on whistleblowers who disclose classified material.
The following day reporters were told there had been but one call in which the two men discussed logistics of a planned conversation between Trump and President Vladimir Putin of Russia.
US Vice President Mike Pence insisted the general and the diplomat did not discuss Washington's sanctions against Russia - an assertion which left Pence looking foolish when it turned out he had been misled.
Flynn was sworn in as national security adviser on January 22. Two days later, he was interviewed by FBI agents about his conversations with Kislyak.
The discussions clearly raised alarm bells because on January 26 Acting Attorney General Sally Yates - since fired by Trump - informed the White House that Flynn had not been frank about the calls and that as a result he was vulnerable to Russian blackmail.
The impression left by Flynn's exit is that of an administration in turmoil as its senior figures jockey for influence.
His departure begs several questions: what else is there to know about contact between White House officials and Moscow? What did the White House do after Yates dropped her bombshell? Was Flynn acting on his own, or was he told to call the ambassador?
Trump has complained that the issue is not Flynn's disgrace but the torrent of leaks making life difficult for his White House.
But the president himself helped foster this crisis by making the enemies of the intelligence and defence communities. His political survival rests on finding an untarnished security adviser and convincing an astonished world that he, and the people around him, really do know what they are doing.
