People can term NATO "fiction" all you want, but these countries entered into that structure with security guarantees.
Which guarantees are worthless.
As the old saying used to go, "With the Russians in Frankfurt, the deutschmark is toilet paper".
There is not a single European state, with the possible exception of Germany, that would shed blood over Estonia.
And, given that Germany is a possible exception, I expect Putin and Merkel to reach an understanding before this goes too much further.