By Robert D. Kaplan and Matt Gertken

Arguably the greatest book on political realism in the 20th century was University of Chicago Professor Hans J. Morgenthau's Politics Among Nations: The Struggle for Power and Peace, published in 1948. In that seminal work, Morgenthau defines the status quo as "the maintenance of the distribution of power that exists at a particular moment in history." In other words, things shall stay as they are. But it is not quite that clear. For as Morgenthau also explains, "the concept of the 'status quo' derives from status quo ante bellum," which, in turn, implies a return to the distribution of power before a war. The war's aggressor shall give up his conquered territory, and everything will return to how it was.

The status quo also connotes the victors' peace: a peace that may be unfair, or even oppressive, but at the same time stands for stability. For a change in the distribution of power, while at times just in a moral sense, simply introduces a measure of instability into the geopolitical equation. And because stability has a moral value all its own, the status quo is sanctified in the international system.

Let us apply this to Asia.

Because Japan was the aggressor in World War II and was vanquished by the U.S. military, it lay prostrate after the war, so that the Pacific Basin became a virtual American naval lake. That was the status quo as it came to be seen. This situation was buttressed by the decades-long reclusiveness of the Pacific's largest and most populous nation: China. Japanese occupation and civil war left China devastated. The rise to power of Mao Zedong's communists in 1949 would keep the country preoccupied with itself for decades as it fell prey to destructive development and political schemes such as the Great Leap Forward and the Cultural Revolution. China was not weak, as the United States would discover in the Korean and Vietnamese wars and later turn to its advantage against the Soviets. But its revolution remained unfinished. The economy did not truly start to develop until the late 1970s, after Mao died. And only in the mid-1990s did China begin its naval expansion in a demonstrable and undeniable way. Thus the United States, in its struggle with the Soviets, got used to a reclusive China and a subordinate Japan. With these two certainties underlying the Cold War's various animosities, the United States preserved calm in its lake.

But the 21st century has not been kind to this status quo, however convenient it may have been for American interests. China's naval, air, cyber and ballistic missile buildup over the past two decades has not yet challenged U.S. military supremacy in the region, but it has encroached significantly on the previously unipolar environment. Moreover, to measure China's progress against U.S. supremacy is to neglect the primary regional balance of power between China and Japan. Tokyo, over the same time period, has come to see China as reaching a sort of critical mass and has accelerated its own military preparations, both in a quantitative and a qualitative sense. Recently, Tokyo has taken to trumpeting its abandonment of quasi-pacifism in order to adjust the world's expectations to what it sees as a new reality. Japan was already a major naval power — it ranks fourth in total naval tonnage, has more destroyers than any navy besides that of the United States, and its technology and traditions give it a special edge. But now it is moving faster to loosen restrictions on its rules of engagement and to upgrade the capabilities it needs to defend its most distant island holdings.