As a deterrent to invasion, use as a super-mine against ground troops would be perfectly adequate and requires no fancy delivery system; just adequate security as to exactly where it was located.
Nope, you fall back so the nuke is behind the enemy's lines; classic ambush tactics. As an example, the Iraqis could have left a nuke buried behind themselves to knock out substantial US forces, if they had one. I haven't seen anything about it, but how much precautions the military took would tell you how seriously the gov't actually took their own claims about Iraqi nukes.
