Michael T. Flynn resigned as national security adviser on Monday night following the revelation that he had misled White House officials about conversations he had with the Russian ambassador to the United States.

Here’s a timeline of his tenure:

— Nov. 18

President-elect Donald Trump offered Flynn the post of national security adviser, elevating the retired general and intelligence officer who saw Islamist militancy as a global existential threat.

“I am pleased that Lt. Gen. Michael Flynn will be by my side as we work to defeat radical Islamic terrorism, navigate geopolitical challenges and keep Americans safe at home and abroad,” Trump said at the time.

— Dec. 29

Former President Barack Obama ejected dozens of suspected Russian intelligence operatives from the United States and imposed sanctions on Russia after U.S. intelligence officials concluded that the nation had ordered the hacking of the Democratic National Committee in an attempt to tip the election to Trump.

In a phone conversation around the same time, Flynn discussed the sanctions with the Russian ambassador, Sergey I. Kislyak, and urged Russia not to retaliate because doing so could complicate future cooperation between the two nations, according to former and current administration officials. The call was captured on a routine wiretap of diplomats’ calls, officials said.

Flynn never explicitly promised sanction relief, but seemed to leave the impression that it was possible, officials said. Private citizens are prohibited by law from engaging in diplomacy.

— Jan. 15

Vice President-elect Mike Pence was asked about the phone call on the CBS Sunday morning program “Face the Nation,” following a Washington Post report on Flynn’s ties to Russia and his conversation with Kislyak.

John Dickerson, the program’s host, asked if the exchanges had served to moderate the Russian response to the sanctions. Pence denied that sanctions had been discussed and described the timing of the conversation as “strictly coincidental.”

“What I can confirm, having spoken to him about it, is that those conversations that happened to occur around the time that the United States took action to expel diplomats had nothing whatsoever to do with those sanctions,” Pence said.

Officials said that Pence now believes that Flynn had lied to him by denying that the sanctions were discussed.

— Jan. 26

Soon after FBI agents interviewed Flynn about the call, the acting attorney general, Sally Q. Yates, told Donald F. McGahn II, the new White House counsel, that Flynn was vulnerable to Russian blackmail due to the discrepancy between his public statements and what intelligence officials knew about the call.

Trump was told “immediately” and instructed McGahn to conduct a review, Sean Spicer, the White press secretary, later explained. The review lasted several days and ended when McGahn concluded that nothing in the conversation had violated federal law.

— Feb. 10

Following reports from several news outlets, Trump said he planned to “look into” Flynn’s conversation with Kislyak.

“I don’t know about that,” Trump told reporters on Air Force One. “I haven’t seen it.” The White House publicly denied that sanctions were discussed.

The White House later said that Trump had been informed weeks earlier that Flynn had not told the truth about his conversation with the Russian ambassador. A former administration official said the Justice Department had issued a warning in January to the White House, raising concerns that Flynn could be vulnerable to blackmail by Moscow.

— Feb. 13

Despite conflicting reports about his status throughout the day, Flynn resigned on Monday evening, less than one month after Trump took office. The resignation came so quickly that at least two staff members from the National Security Council said they learned about it from news reports.

“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.”

“I am tendering my resignation, honored to have served our nation and the American people in such a distinguished way,” he wrote.

Lt. Gen. Joseph K. Kellogg Jr. was named acting national security adviser.

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