“And these terrorists would be unbelieveably lucky to engineer a 12-18 kT explosion - 0.5-1.0 kT is much more like it.”
That’s presuming they don’t get their hands on one of the dozens of missing weapons mentioned in the articles.
A small Soviet fusion weapon is in the 1 MT range.
My nightmare scenario doesn’t involve carrying things over the border on foot. It involves a cargo ship pulling into a busy port, with a Navy base (say, San Diego)...and suddenly the cargo ship is replaced by a 1+ MT fireball.
That would be a bad day.