Of course, he can’t actually prove either claim to be true, and he concedes as much. Instead, he bases his case on a particular set of beliefs about how the world works and about the United States’ unique characteristics within that system. Kagan asserts that the world requires a single, order-inducing hegemon to enforce the rules of the game and that America must perform this role because its global economic interests demand it. He also believes that the United States has a special obligation, deriving from its heritage as a “dangerous nation,” to spread democracy and human rights. What’s more, America’s military might is the essential ingredient that leads to its international influence. The spread of democracy and market capitalism, Kagan claims, is made possible by U.S. power but would retreat before autocracy and mercantilism if that power were seen to be waning. The attractiveness of America’s culture, economics and political system—the vaunted “soft power” in Joseph Nye’s telling—is fleeting and would dissipate if Americans were to commit what Kagan calls “preemptive superpower suicide.”

How other nations respond to U.S. power also follows a familiar pattern. In Kagan’s telling, allies will bandwagon with us if we are committed to defending them but bolt like frightened racehorses at the first sign of trouble. Would-be challengers will back down in the face of U.S. power but rush to exploit opportunities for conquest if Uncle Sam exhibits any hesitation or self-doubt. And Kagan simply dismisses any suggestion that other countries might chafe at American dominance or fear American power.

His ideas represent something close to the reigning orthodoxy in Washington today and for the past two decades. Inside the Beltway, there is broad, bipartisan agreement on the basic parameters of U.S. foreign policy that Kagan spells out. This consensus contends that the burden of proof is on those who argue against the status quo. The United States and the world have enjoyed an unprecedented stretch of security and prosperity; it would be the height of folly, the foreign-policy establishment asserts, to upend the current structure on the assumption that an alternative approach would represent any improvement.

But such arguments combine the most elementary of post hoc fallacies with unwarranted assumptions and idle speculation. Correlation does not prove causation. There are many factors that could explain the relative peace of the past half century. Kagan surveys them all—including economic interdependence, evolving norms governing the use of force and the existence of nuclear weapons—and concludes that U.S. power is the only decisive one. But, once again, he concedes that he cannot prove it.

EVEN THOSE inclined to believe Kagan’s assessment of the international system and America’s role in it must contend with one central fact that Kagan elides: the costs of maintaining the status quo are substantial and likely to grow. That is because Washington’s possession of vast stores of power—and its willingness to use that power on behalf of others—has created an entire class of nations that are unwilling to defend themselves and their interests from threats. The data clearly show a vast and growing gap between what others pay for defense and what Americans pay to defend them.

The critical question, then, centers on differing perceptions of this capability imbalance. Because U.S. security guarantees to wealthy allies have caused them to underprovide for their own defense, they also have less capacity to help the United States in its time of need—either now in Afghanistan or in a theoretical future contest with China or a resurgent Russia.

Kagan contends other countries will choose not to defend themselves and their interests, but at other times he acknowledges it is precisely the presence of American power that has discouraged them from doing so. In the end, it is clear Kagan doesn’t want other countries to defend themselves because, he says, they just can’t be trusted to get the job done. Most will be content to let security challenges grow and fester on their borders, or within them, leaving the United States—and the United States alone—with the task of cleaning up the mess. As he sought to explain in 2003, Americans should “be more worried about a conflagration on the Asian subcontinent or in the Middle East or in Russia than the Europeans, who live so much closer,” because the harm from other countries’ failure to act will inevitably threaten U.S. security.

Kagan correctly argues that the United States could afford to shoulder the burdens of defending others. The costs of U.S. foreign policy are neither insurmountable nor unprecedented. But his case that we should do so is marred by games he plays with statistics in a transparent bid to strengthen his argument.

For example, citing figures compiled by the Department of Agriculture’s Economic Research Service, he argues that the U.S. share of global GDP “has held remarkably steady, not only over the past decade, but over the past four decades.” In fact, the U.S. share of global GDP is declining, though modestly. The USDA data set clearly shows that the U.S. share of global output peaked during the post–Cold War period at 28.38 percent in 1999 and has since fallen to 25.48 percent. During that same period, China’s share rose from 3.44 percent to just over 8 percent.

A different data set prepared by the International Monetary Fund paints a more vivid picture of America’s relative decline. From a post–Cold War era peak of 32.1 percent in 2001, the U.S. share of global GDP had fallen to 21.7 percent by 2011 and was projected to fall to 21 percent by 2017.

More telling still are the statistics showing how the U.S. share of global military spending has risen from less than 30 percent at the end of the Cold War to nearly 48 percent today. Per capita spending exhibits a similar trend. According to data from the International Institute for Strategic Studies’ annual reports, The Military Balance, in 2010 Americans spent 71 percent more (in inflation-adjusted terms) on national security than they did in 1998, whereas other countries have either increased spending only slightly or reduced their spending in real terms over that same period.

Taken together, these trends undermine Kagan’s contention that U.S. military spending has not grown more burdensome for American taxpayers. Still, as a purely economic matter, such spending isn’t unsustainable. The U.S. federal debt and deficit don’t require Americans to adopt a more restrained foreign policy. An age of austerity in Washington, should it ever come to pass, would not necessarily translate into a smaller military with fewer missions.

But such a shift would be wise on its merits. A world order that was less dependent on U.S. military power would likely result in a greater number of countries with more military capability and a greater willingness to use it. Most Americans, contrary to Kagan’s claims, would welcome this if it meant a more manageable burden for America.

Kagan asserts that despite “their misgivings, most Americans have also developed a degree of satisfaction in their special role.” Yet polling data show precisely the opposite: most Americans want desperately for others to shoulder the burdens of defending themselves and their interests. For example, 79 percent of voters told pollster Scott Rasmussen that we spend too much money defending others; a mere 4 percent think we don’t spend enough. A CNN survey last year found that just one in four Americans relished the United States’ being the world’s “policeman,” and a separate Rasmussen poll concluded that a mere 11 percent of likely voters support that mission.

But Kagan and other advocates of U.S. benevolent global hegemony contend Americans must play this role of global policeman. It would be irresponsible, they say, to stake the future of the present world order on the supposition that other countries would assume some of the burdens of global governance that Americans shed. Kagan assumes other countries would not because they have not done so since at least the end of the Cold War. But this ignores the extent to which U.S. foreign policy—Kagan’s foreign policy—has discouraged them from doing so, a point he regularly celebrates. He points especially to Germany and Japan, whose choices not to rearm after World War II were heavily influenced by Washington. “Had the American variable been absent,” he concludes, “the outcome would have been different.”

This expansive global role that Kagan champions may have made sense during the early days of the Cold War, when the countries of Western Europe and East Asia were shattered and we were confronting a common enemy. But the world has changed. The strategy Kagan advocates has needlessly and unfairly burdened Americans with the costs of maintaining global peace, and it could—and should—have been altered long ago. Yet if Robert Kagan has his way, it never will be.

AS NOTED above, Kagan is anxious to emphasize that our military power is not particularly costly. His preferred technique for demonstrating this fact is to refer to military spending as a share of GDP. And it is true that the United States has in the past sustained a higher level of military spending when measured against the nation’s total output; likewise, military spending as a share of the federal budget has traditionally been far higher than it is today.

Pullquote: The world is more complicated and durable than Kagan imagines. The United States does not need to police the globe in order to maintain a level of security that prior generations would envy.Image: Essay Types: Book Review