WASHINGTON ― Michael Flynn, an early and enthusiastic backer of President Donald Trump, resigned from his post as national security adviser on Monday evening, after just 25 days on the job.

It was the second time in less than three years that the former intelligence analyst was forced out of a prestigious government position. In 2014, Flynn, a lieutenant general at the time, announced that he would step down as director of the Defense Intelligence Agency about a year before he was expected to leave. DIA officials have said he was forced out because of his “chaotic” management style, but Flynn has said he believes that the real issue was that he raised the alarm about the threat of “radical Islamism.”

Trump’s presidency offered a second chance for the ousted lieutenant general ― and this time with a commander in chief who would take seriously his warnings about Islamic terrorism. But before even entering the White House, Flynn made a mistake that would ultimately cost him his job in the Trump administration.

Congressional Democrats began calling for Flynn’s resignation after a series of leaks revealed that he spoke with the Russian ambassador to the U.S. over the phone on Dec. 29 ― the same day that the Obama administration imposed sanctions against Russia in response to its alleged effort to help Trump win the election through a series of cyberattacks. Flynn initially said that he didn’t discuss the sanctions with Sergey Kislyak, but later backed away from that denial after it became clear that the FBI had transcripts of the conversations.

Here’s what we know about Flynn’s ties to Moscow and his interactions with Kislyak:

Dec. 10, 2015: Flynn traveled to Moscow for a paid speaking gig at a celebration of the 10th anniversary of Russia Today, a state-owned news network that critics characterize as a propaganda outlet for the government. Flynn was photographed sitting next to Russian President Vladimir Putin during the trip. The Army is reportedly investigating whether Flynn received money from the Russian government at the time, which could violate the emoluments clause of the Constitution.

Sputnik Photo Agency/Reuters Lt. Gen. Michael Flynn seated next to Russian President Vladimir Putin in Moscow on Dec. 10, 2015.

Nov. 18, 2016: Trump chose Flynn to serve as his national security adviser, a role that guarantees Flynn access to the country’s most closely guarded secrets and a direct line to the president.

Dec. 29, 2016: President Barack Obama announced sanctions against Russian intelligence services and expelled 35 Russian diplomats from the U.S.

Flynn and Kislyak spoke several times by phone, The Washington Post later reported.

Dec. 30, 2016: Russian Foreign Minister Sergey Lavrov suggested his country would retaliate. “We, of course, cannot leave unanswered the insults of the kind, reciprocity is the law of diplomacy and foreign relations,” he said. The Foreign Ministry recommended Putin expel 31 U.S. diplomats from Moscow and four from St. Petersburg.

Putin issued an official statement, announcing that Moscow would not retaliate. Instead, Putin said, they would wait to work with the incoming Trump administration to restore relations.

Trump congratulated Putin on the decision.

Great move on delay (by V. Putin) - I always knew he was very smart! — Donald J. Trump (@realDonaldTrump) December 30, 2016

Late December 2016/early January 2017: Intelligence analysts looked for clues to explain Moscow’s unexpected reversal. That search led them to transcripts of intercepted conversations between Flynn and Kislyak ― whose communications are monitored by the FBI.

Jan. 12, 2017: Eight days before Trump’s inauguration, David Ignatius of The Washington Post reported the Dec. 29 phone calls between Flynn and Kislyak. Ignatius noted that the calls occurred on the same day as the new sanctions against Russia, but did not describe the contents of the calls. He later updated the article to include a Trump transition official’s claim that the calls took place before the sanctions were announced.

Jan. 13, 2017: Incoming White House press secretary Sean Spicer told reporters in a conference call that the conversation between Flynn and Kislyak “centered on the logistics” of setting up a post-election call between Trump and Putin. “That was it, plain and simple,” Spicer said.

Jan. 15, 2017: Justice Department officials discussed whether the White House should be told about the contents of the conversations between Flynn and Kislyak.

Amid questions about whether Flynn had undercut Obama’s sanctions in conversations with Kislyak, Vice President-elect Mike Pence said on CBS’s “Face the Nation” that Flynn “did not discuss anything having to do with the United States’ decision to expel diplomats or impose censure against Russia.” The timing of the phone calls was “strictly coincidental,” Pence said.

Jan. 19, 2017: Deputy Attorney General Sally Yates, outgoing Director of National Intelligence James Clapper, and outgoing CIA Director John Brennan argued in favor of telling Trump about the Flynn-Kislyak communications, The Washington Post reported. FBI Director James Comey opposed telling Trump, fearing it could interfere with an ongoing investigation into ties between Trump associates and Russia.

Jan. 23, 2017: Spicer told reporters Flynn and Kislyak had one phone conversation. It covered a plane crash that killed members of a Russian military choir, Christmas greetings, negotiations related to the civil war in Syria and setting up the Trump-Putin phone call.

Yates pressed Comey again about notifying the White House. By this time, his opposition had faded.

Saul Loeb/Getty Images White House press secretary Sean Spicer told reporters on Jan. 23 that Flynn did not discuss sanctions against Russia with Kislyak.

Jan. 26, 2017: Yates briefed White House counsel Donald McGahn about Flynn’s discussion with Kislyak, Spicer told reporters Tuesday. The Washington Post previously reported that Yates and a senior career national security official told McGahn that they believed Flynn had misled senior administration officials about the content of the conversations. They reportedly warned that Flynn may have placed himself in a position in which he could be vulnerable to blackmail by Moscow.

Feb. 8, 2017: Flynn categorically denied discussing sanctions with Kislyak during an interview with the Post.

Feb. 9, 2017: Through a spokesman, Flynn told the Post he “couldn’t be certain that the topic never came up.”

The Post reported that Flynn and Kislyak discussed sanctions in December. “Kislyak was left with the impression that the sanctions would be revisited at a later time,” a former official told the Post.

Feb. 10, 2017: The Kremlin confirms that Flynn and Kislyak spoke by phone, but denies that they discussed sanctions.

Trump told reporters aboard Air Force One en route to the Mar-a-Lago estate in Florida that he was unaware of the Post report that quoted former and current U.S. officials accusing Flynn of misleading the administration about his conversations with Kislyak. “I haven’t seen that. I’ll look into that,” Trump said.

Flynn called Pence to apologize for misleading him about conversations with Kislyak, ABC later reported.

Rep. Adam Schiff (Calif.), the top Democrat on the House Intelligence Committee, called for Flynn’s removal. “The allegation that General Flynn, while President Obama was still in office, secretly discussed with Russia’s ambassador ways to undermine the sanctions levied against Russia for its interference in the Presidential election on Donald Trump’s behalf, raises serious questions of legality and fitness for office,” Schiff said in a statement.

Feb. 12, 2017: White House policy adviser Stephen Miller dodged questions about Flynn during interviews on the Sunday morning talk shows. Asked if the president still has confidence in his national security adviser, Miller told NBC, “That’s the question that I think you should ask the president, the question you should ask Reince [Priebus], the chief of staff.”

Feb. 13, 2017: House Minority Leader Nancy Pelosi (D-Calif.) called on Flynn to be “fired immediately.” He “cannot be trusted not to put Putin before America,” she said in a statement.

Kellyanne Conway, counselor to Trump, told MSNBC that Flynn has “the full confidence of the president.” Priebus said “the president is evaluating the situation” in a statement released an hour later.

Flynn submitted his resignation letter later that night. He admitted to mischaracterizing his phone calls with Kislyak, but chalked the incident up to an accident.

“Unfortunately, because of the fast pace of events, I inadvertently briefed the Vice President Elect and others with incomplete information regarding my phone calls with the Russian Ambassador,” Flynn wrote in his resignation letter. “I have sincerely apologized to the President and the Vice President, and they have accepted my apology.”