Most trends on the Korean Peninsula favor South Korea, but North Korea's nuclear program is a great concern, as is the possibility that the North will become desperate at some point because of negative trends. Although unlikely, war — even limited nuclear war — is imaginable in the years ahead, perhaps with winners and losers. This poses challenges for strategic planning. RAND and KIDA have been conducting a collaborative research addressing this issue and this paper is the output of the first year's research. The first section of this paper reviews and extends strategic theory and lessons from the Cold War by drawing on classic papers and more recent literature, the principal author's experiences with U.S. strategic planning, recently declassified materials, history, and psychological research. This is followed by a section that outlines the challenges for deterrence salient to Korea today, particularly ways in which deterrence could fail, using historical cases. The final section discusses implications for Korea, highlighting the need to think through how South Korea should consider its new military capabilities and the need to strengthen U.S. extended deterrence. Complacency regarding both matters would be dangerous. Deterrence could fail for such reasons as misperceptions, misunderstanding the adversary, other aspects of limited rationality, and accidents. Further, the challenges for extended deterrence are much greater than earlier, as are those in achieving balance in planning.