Is interstate war becoming obsolete? Many thoughtful observers, including political scientists, now believe so. Six key arguments—not to mention the modest number of interstate wars in recent decades—support their thesis. The nuclear revolution has made such conflict between major powers far more foreboding. The memory of the world wars and an understanding of the destructiveness of industrial-age conventional weaponry have further chastened any global leader who might contemplate acts of aggression. The US-led Western alliance system has also made the international political environment less anarchic. The spread of democracies, which have tended not to fight each other, has added a further stabilizing element. Modern economics has made the acquisition of additional territory less important for enhancing national wealth than in previous eras. In any event, anti-colonial movements of the 20th century made imperial conquest far more difficult politically and militarily. These six major developments in human history suggest that the 21st century will involve far less violence between countries than did the 20th century—even if civil conflict, terrorism, and other nontraditional threats to security remain serious worries.

On the whole, these broad assessments about trends in human conflict seem correct. For the foreseeable future, international acts of violence appear much more likely to arise at the intrastate or nonstate level than between countries. Conflicts between major powers seem particularly unlikely. However, it would be a major intellectual error—and quite possibly a tragic policy mistake—to discount the possibility of war between the international community’s larger actors. Those who point to the declining frequency of interstate wars sometimes forget that such conflicts have rarely been numerous. Their prevalence matters less than their severity once they do unfold. In addition, there has been no continual decline in their frequency over the past half-century, and recent years have witnessed a slight upturn in their number, according to a recent analysis by Ted Gurr, Monty Marshall, and Deepa Khosla of the University of Maryland.

Of the six major factors listed above that make countries less apt to go to war with each other, not all are absolute. Memories of World War I, World War II, and the Korean and Vietnam conflicts will fade with the passing of the generations that waged them. The US-led alliance system, remarkably successful by the standards of human history and still quite strong today, could face severe challenges in the years ahead. Given the absence of a clear external threat, and in the face of disagreements between Washington and many allied capitals over issues such as Iraq, Iran, Taiwan, and national missile defense, the alliance system’s cohesion cannot be presumed to last forever.

Of the remaining four global realities discouraging interstate war, each has its limitations. As for nuclear deterrence, it certainly reduces the risk that countries will march on each other’s capitals. It is less obvious, however, that nuclear deterrence is reliable for preventing wars over more limited stakes such as Kashmir or Taiwan. Democracies do tend to fight each other less often—once they are constitutionally and politically consolidated, and once they fully perceive each other as democracies, as scholars such as Edward Mansfield, Jack Snyder, and John Owen have argued. Democracies in transition have far less stabilizing effects. Moreover, countries that become democratic may not always stay that way, as the example of Weimar Germany reminds us. Colonialism may be dead, but irredentism is not, and some countries still reserve the right to use force to defend or acquire disputed lands. Finally, modern high-technology market economics may reduce the need for large swaths of land to assure national power and prestige. But key resources remain as critical as ever to the success of nations’ economic systems, and it is not difficult to imagine wars driven by governments’ desires to secure such resources.

These general concerns may be fleshed out by thinking through several future conflict scenarios that seem plausible—or at least difficult to dismiss out of hand.

Cold Legacy: Korea

It may only represent a small appendage to the giant Eurasian land mass, and the military confrontation it hosts may seem little more than a leftover conflict from the Cold War. But the Korean peninsula remains dangerous: any war there could have consequences transcending what is directly at stake.

Promising moves toward détente notwithstanding, the peninsula remains the most militarized place on earth. The density of forces on both sides of the demilitarized zone is greater than the concentration of the North Atlantic Treaty Organization’s (NATO) and Warsaw Pact’s armed units along the intra-German border during the Cold War. Worse, there are still plausible paths to violence. For example, if Pyongyang attempts to coerce more aid out of the West with its missile and nuclear programs, but Seoul, Washington, and Tokyo respond in a hard-line fashion, the situation could deteriorate. That sequence of events could lead to a joint US-South Korean air strike against key North Korean nuclear or missile facilities. That in turn could provoke a North Korean response, up to and including another attempt to invade South Korea. Each of these steps is in itself unlikely, and together they are quite unlikely. But this scenario is one that is far from unthinkable.

The United States and South Korea would surely prevail in any such war. However, there could nonetheless be serious consequences for the broader international system. Notably, if thousands of US soldiers were killed in the conflict, and the war was seen in part as the result of bungling by Western leaders rather than that of blatant North Korean aggression, the US public could lose much of its interest in maintaining a global system of alliances underwritten by its tax dollars and by the lives of its troops. At present, the US public supports the United States’ strong role in the world partly because it believes in it, but also because international engagement has not caused a massive loss of US troops for decades. Should that fact change in a post-Cold War setting, the effects upon the US public would be unpredictable and possibly harmful to sustaining a strong security community of allied democratic nations.