National security adviser Michael Flynn resigned from his position late Monday after his phone call with Sergey Kislyak, the Russian Ambassador to Washington, D.C., about sanctions the Obama administration had levied.

Flynn had allegedly misled the White House over the topics of discussion on the phone calls, which took place on Dec. 29, before President Trump's inauguration. The news of the call broke on Jan. 12 in the Washington Post.

Sally Yates, former attorney general who was fired by Trump over his immigration order, notified the White House counsel of the phone call on Jan. 26, said Robert Shrum, director of the USC Unruh Institute of Politics.

At the daily White House press briefing this Tuesday, Sean Spicer said that the president and the White House counsel decided that Flynn's talks with Kislyak were legal. "There was not an issue of law, there was an issue of trust," Spicer said.

He also said the leak should be investigated, though Shrum said "searching for leaks is a feudal venture."

Shrum said Flynn should have never been nominated as national security advisor in the first place, and should have been more careful in his call. "He should have known that that call would most likely had been intercepted by the CIA or the NSA or the FBI," Shrum said. "He shouldn't have made it in the first place."

President Obama dismissed Flynn in 2014 as director of Defense Intelligence Agency "because he could not get along with the people he was supposed to work with," Shrum said. Flynn also made statements and assertions which were inaccurate — "Flynn facts" — as the DIA subordinates termed it — along with starting a chant of "Lock her up!" at the Republican National Convention.

Flynn was also paid for a speech he gave for the tenth anniversary of the Russian Today radio broadcast, according to Shrum. "There should have been red flags," he said.

"He got the job with his warm up acts at a bunch of [Trump] rallies," Shrum said. "He basically got hired because he was very, very loyal."

The U.S. military investigated Flynn in 2010, after receiving a complaint by a source in the Navy who wished to stay anonymous. The complaint stated that Flynn repeatedly violated rules by sharing secrets with "various foreign military officers and/or officials in Afghanistan."

In Michael Flynn's resignation letter, he said that his apology to the President and Vice President were accepted. "Unfortunately, due to 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," he said in the letter.

Prior to Flynn's resignation, Sen. Marco Rubio (R-FL) said there should be an extended investigation on the alleged Russian meddling with the presidential elections.

Ranking Democratic member of the House Intelligence Committee, Adam Schiff, indicated how the Trump administration has been cryptic about who knew what Flynn said and when they heard it. "The Trump administration has yet to be forthcoming about who was aware of Flynn's conversations with the ambassador and whether he was acting on the instructions of the president or any other officials, or with their knowledge," Schiff said.

Flynn's resignation is unique, Shrum said. "The national security advisor lasting about three weeks into an administration is unprecedented."