The “stopping power of water” is where Tragedy, in an analytical sense, builds toward its powerful conclusion. “Large bodies of water are formidable obstacles that cause significant power-projection problems,” Mearsheimer writes. Great navies and air forces can be built, and soldiers transported to beachheads and airstrips, but conquering great land powers across the seas is difficult. This is why the United States and the United Kingdom have rarely been invaded by other great powers. It is also why the U.S. has almost never tried to permanently conquer territory in Europe or Asia, and why the United Kingdom has never tried to dominate continental Europe. Therefore, the “central aim of American foreign policy” is “to be the hegemon in the Western Hemisphere” only, and to prevent the rise of a similar hegemon in the Eastern Hemisphere. In turn, the proper role for the United States is as an “offshore balancer,” balancing against the rise of a Eurasian hegemon and going to war only as a last resort to thwart it. But better to try buck-passing first, Mearsheimer advises, and come into a war only at the last moment, when absolutely necessary.

Mearsheimer tells me that the U.S. was right to enter World War II very late; that way it paid a smaller “blood price” than the Soviet Union. “Before D-Day, 93 percent of all German casualties had occurred on the eastern front,” he says, adding that the devastation of the Soviet Union helped the U.S. in the Cold War to follow.

“How is offshore balancing different from neo-isolationism?,” I ask him. “Isolationists,” he responds, “believe that there is no place outside of the Western Hemisphere to which it is worth deploying our troops. But offshore balancers believe there are three critical areas that no other hegemon should be allowed to dominate: Europe, the Persian Gulf, and Northeast Asia. Thus,” he goes on, “it was important to fight Nazi Germany and Japan in World War II. American history suits us to be offshore balancers—not isolationists, not the world’s sheriff.” Later, when I ask Mearsheimer about the Obama administration’s slightly standoffish policies toward Libya and whether they are a good example of buck-passing, he says the problem with leading from behind in this case was that America’s European allies lacked the military capacity to do the job efficiently. “If mass murder was truly in the offing, as it was in Rwanda,” he tells me, “then I would have been willing to intervene in Libya. But it is unclear that was the case.”

Such thinking is prologue to Mearsheimer’s admonition that a struggle with China awaits us. “The Chinese are good offensive realists, so they will seek hegemony in Asia,” he tells me, paraphrasing the conclusion to Tragedy. China is not a status quo power. It will seek to dominate the South China Sea as the U.S. has dominated the Greater Caribbean Basin. He continues: “An increasingly powerful China is likely to try to push the U.S. out of Asia, much the way the U.S. pushed European powers out of the Western Hemisphere. Why should we expect China to act any differently than the United States did? Are they more principled than we are? More ethical? Less nationalistic?” On the penultimate page of Tragedy, he warns:

Neither Wilhelmine Germany, nor imperial Japan, nor Nazi Germany, nor the Soviet Union had nearly as much latent power as the United States had during their confrontations … But if China were to become a giant Hong Kong, it would probably have somewhere on the order of four times as much latent power as the United States does, allowing China to gain a decisive military advantage over the United States.

Ten years after those lines were written, China’s economy has passed Japan’s as the world’s second-largest. Its total defense spending in 2009 was $150 billion, compared with only $17 billion in 2001. But even more revealing is the pattern of China’s military modernization. “Force planning—the product of long-term commitments and resource allocation decisions—is the heart of strategy,” the military expert Thomas Donnelly, of the American Enterprise Institute, wrote last year. And for more than a decade now, China’s military

has shifted its focus from repelling a Soviet invasion and controlling domestic unrest to the sole problem of defeating U.S. forces in East Asia. This has been a strategic surprise to which no American administration has appropriately responded.

China is increasing its submarine fleet from 62 to 77 and has tested a stealth fighter jet as part of a buildup also featuring surface warships, missiles, and cyber warfare. Andrew F. Krepinevich, the president of the Center for Strategic and Budgetary Assessments, believes that nations of the Western Pacific are slowly being “Finlandized” by China: they will maintain nominal independence but in the end may abide by foreign-policy rules set by Beijing. And the more the United States is distracted by the Middle East, the more it hastens this impending reality in East Asia, which is the geographical heart of the global economy and of the world’s navies and air forces.