[justin66]

That's trickier than you'd think. You either have to send the warheads one well after the other, or you have to set them up so they detonate at precisely the same time, so as to avoid the first warhead to detonate destroying all the others. (This is why, at one time, missile silos were placed relatively close together. The enemy could presumably get one of them in a preemptive strike, but not all of them.)

At Korea's level of technology, for example, you'd probably just pick a secondary target in some other place entirely.