The twenty-first century has brought new challenges. Climate change may be the most dangerous in the long run but new forms of pandemic illness may be a more frequent occurrence now. A more traditional kind of threat has also sharpened, great-power rivalry. Adding to this threat will be the continuing spread of dangerous technologies to smaller nations and private groups.
Great-power rivalry will continue as long as national interests take priority over global ones. However, the last two centuries of modern development have brought different nations and cultures closer together by compressing distance. Daily life for most people has become more stable than when natural conditions made survival less assured, but new uncertainties over market forces, military tensions, and political conditions have taken their place.
Planning for national security tends to assume that great-power rivalry will continue indefinitely, as it has in the past. However, this assumption may no longer hold if an end-state is foreseeable. An end-state can be foreseen if the future of great-power rivalry lies in outer space. No great power can allow another one to dominate outer space and any power that does so will dominate the world.
Arms control may be proposed to limit competition in space. However, we live in a world where one group of powers (China with Russian support) is opposed by another group (America and its allies). Since the China group aims to revise the balance of power in its favor, arms control that freezes the balance is unlikely to be acceptable to both sides. If great-power rivalry continues and extends into outer space, a third world war is probably inevitable.
America and its allies will have a choice to make if they survive and prevail after a new world war. They may do no more than they did when the Cold War ended in 1991. Or they may try to achieve a world order very different from the one that evolved after 1991. This choice should be a subject of debate before a world war breaks out, as a better vision might shorten a conflict or possibly even prevent one.