How would the prospect of global nuclear war change if missile defense systems grew in technology?

The W54 (and, by extension, its variants), a small nuclear weapon, easy to move and hide, with a .19 to 1kt yield.

3,000 miles of Canadian border.
You can drive a bulldozer across it and no one would notice for days.
Here's my favorite example:
https://www.google.com/maps/@49.002...5tAEl-A2W35wpVGiIA!2e0!7i13312!8i6656!6m1!1e1
The border is marked by the post and runs along the far side of the ditch.

 

Well, if you are so paranoid, how about petitioning the government then to put up a wall to keep out those Pesky Canadians.

How dare they invade us with Justin Beiber and William Shatner.

 

You miss the point.
Smuggling such a thing into the US, for any organization capable of getting a hold of one, won't be much of an issue.