"On the other hand, if the potential foe thought that we might not retaliate once the threat served no purpose—that retaliatory "deterrence" would, in essence, turn out to be a bluff—it would encourage those disposed to strike first to cause a nuclear holocaust without fear of reprisal. We had to threaten genocide—and convince people we meant to carry out our threat—in order to prevent genocide."

Well, that's what that letter is about. Not having to build such behaviour into the system while remaining ambiguous about the threat.