Before the modern era, most nations didn't spend much time speculating about where their next war would be or who it would involve. Geography largely determined who would fight whom. With the rare exception of invaders from afar, enemies often remained at each other's throats for decades, even centuries. States knew who they would fight -- the only question was when. But the United States is different. With no major enemies nearby, America's wars have been fought around the world against a wide range of opponents. This meant that U.S. policymakers and military leaders needed to anticipate the location and identity of their enemies. Being wrong had strategic and blood costs.
Unfortunately, the United States isn't particularly good at anticipating where its next war will be and what enemy it will face. Most have been fought in places and against enemies that received little attention before America became involved. The wars that the United States did plan meticulously -- a conflict in Europe between NATO and the Warsaw Pact, and a second Korean War -- never happened. This pattern of fighting unexpected wars should tell us something: Rather than picking today's most hostile opponent, whether North Korea, Iran or an increasingly aggressive China, and designing the future U.S. military to fight it, America should think more broadly and creatively about the type of wars it might face in the future.
Changes underway in the global security environment suggest that America's next war will be one of six types. The most obvious scenario is one where provocation and nuclear proliferation compel the United States to destroy the military capability of a hostile state and possibly change its regime. North Korea and Iran are, of course, the leading candidates in this category, but others are likely to emerge in the coming decades. If North Korea can build nuclear weapons, many other nations can as well. And aggressive, miscalculating dictators are never in short supply. Wherever it occurs, this type of war could drag the U.S. military into a long occupation, since simply destroying existing nuclear weapons or missiles may be only a temporary fix. So what begins as a conventional operation could end up as a counterinsurgency campaign, potentially a large and long one.