I had a look around the site and couldn't find this mentioned. I read it as a young teenager and it terrified me and blew my mind.
Level 7, by Mordecai Roshwald.
The protagonist and narrator (in the style of a personal journal) is known only as X127. He (or possibly she, though personally I feel X127 is a man) is a Push Button Officer, whose job is to push the buttons that launch the weapons that destroy an enemy. We never discover which 'side' X127 is on, and the character themselves assumes they have a counterpart.
It's dark and bleak, and is certainly one of the most direct, articulate and savage indictments of Mutually-Assured Destruction we have all, broadly, been living under since .. well, since 1962, when that phrase was coined (though this book was published in 1959)
No spoilers, except to say there isn't exactly a happy ending. But it really is a remarkable piece of work.