FILE - In this Feb. 11, 2014, file photo, then-Defense Intelligence Agency Director Lt. Gen. Michael Flynn testifies on Capitol Hill in Washington. Flynn resigned as President Donald Trump's national security adviser Monday, Feb. 13, 2017. (AP Photo/Lauren Victoria Burke, File)
1. The background: According to The Washington Post, Flynn "privately discussed U.S. sanctions against Russia" with Ambassador Sergey Kislyak in December while President Barack Obama was still in office, then later misled Trump administration officials – including Vice President Mike Pence – about the conversations. As late as last week, Flynn denied talking to the ambassador about sanctions, but walked back that claim Thursday when a spokesman told the Post that Flynn "couldn't be certain that the topic never came up." A transcript of Flynn's call with Kislyak revealed that the two had spoken about sanctions, The New York Times reported.
2. The resignation: The White House released Flynn's resignation letter Monday night. In the letter, Flynn said he "inadvertently briefed the vice president-elect and others with incomplete information regarding (his) phone calls with the Russian ambassador." He added that he has "sincerely apologized to the president and the vice president, and they have accepted my apology."
3. The warning:The Post reported Monday that former acting Attorney General Sally Yates, who was fired last month after she ordered Justice Department lawyers to stop defending the president's travel ban, told the Trump administration in January that Flynn "was potentially vulnerable to Russian blackmail" over his conversations with the ambassador.
5. What's next? Trump must decide on a permanent replacement for Flynn. According to The Associated Press, the top contenders are Kellogg; former CIA director David Petraeus, the retired four-star general who stepped down after sharing classified information with his biographer-turned-mistress; and retired Vice Admiral Robert Harward, a former deputy commander of U.S. Central Command and member of the National Security Council under President George W. Bush. The Times reported that Harward "is the leading candidate."