I first read this in 1973 or 74. At the time, I thought it "pulpy" as another Goodreads reviewer put it, but not bad. It was believable, but unlikely.
The US decides it needs Canada's Natural Gas reserves, and will go to any lengths to get them. Canada says "you just can't take them, but we'll cut you a really good deal (even after you screw up so badly that anyone else would say you'd blown your last chance), because we're Nice People."
Now, reading it more critically, I think the writing is perhaps better and unfortunately the story (replace Gas with Water) is getting more believable by the minute.
Canada is not the 51st state, and Wayne Gretzky and Elon Musk have both committed treason.