Flynn's 24-day tenure as National Security Advisor was the shortest in the 63-year history of the office.
Those communications he had with the Russian ambassador were subsequently leaked to the press.
Commenting on Flynn's resignation, on February 14 White House Press Secretary Sean Spicer stated, "We got to a point not based on a legal issue, but based on a trust issue, where a level of trust between the President and General Flynn had eroded to the point where he felt he had to make a change ...
The issue here was that the President [Trump] got to the point where General Flynn's relationship misleading the Vice President and others, or the possibility that he had forgotten critical details of this important conversation had created a critical mass and an unsustainable situation.
That's why the President decided to ask for his resignation, and he got it."
The Russia Hoax was originally aimed at Flynn, not Trump.
I am no Flynn fan but they way he was pushed out by Pence smacks of palace intrigue. Pence is a loyal GOPe and not one of the MAGA good guys, IMO. Flynn was with POTUS way before Pence parachuted in and was one of the few early supporters given a major role in the Trump admin.