Trump is going to increase the military anyway, and so are all of the NATO allies. They already are. Putin’s greatly alarmed even the neutral nations Europe. The collective wealth and potential power of all these nations is formidable, regardless of what people may think about the shirtless Putin he would not want to wage war on all of Europe.
a) there is no FBI investigation-- routine eavesdropping intercepted Flynn talking to the Russian ambassador;
b) all incoming NSA's talk to the Great Powers;
c) the media is lying about a purported investigation;
d) Flynn may have misspoken, but did not deliberately misinform anyone;
e) Flynn has my full confidence and will remain as NSA
Trump didn't do this, so I suspect there's more to it. My guess is (d) is incorrect-- Flynn misled them about something.