The second major U.S. relative decline occurred from 1970 to 1980, when the U.S. share of world product fell 27 percent. This decade brought with it challenges to America's position in the world. This was especially true toward the end of the decade with the Soviet invasion of Afghanistan and the Iranian Revolution, which collectively increased concern about Soviet dominance of Persian Gulf oil. However, the 1970s was mainly a period of "détente" between the cold-war protagonists, which corresponds to the fact that the shares of world product for both the United States and the Soviet Union were in decline. In other words, it is reasonable to think that America's decline in the 1970s did not lead to more significant trouble for the United States because its main rival was descending even faster.

Clearly, major shifts in the balance of power in the international system often lead to instability and conflict. And America's current predicament is far more severe. This time, our relative decline of 32 percent is accompanied, not by an even-steeper decline of our near-peer competitor, but rather by a 144 percent increase in China's relative position. Further, the rapid spread of technology and technological breakthroughs means that one great discovery does not buoy an already-strong state to decades-long predominance. And with a rising China-with raw resources of population, landmass and increasing adoption of leading technology-a true peer competitor is looming. America's current, rapid domestic economic decline is merely accelerating our own downfall.

The distinct quality of a system with only one superpower is that no other single state is powerful enough to balance against it. A true global hegemon is more powerful still-stronger than all second-ranked powers acting as members of a counterbalancing coalition seeking to contain the unipolar leader. By these standards, America's relative decline is fundamentally changing international politics, and is fundamentally different from Russia circa 1850 and Great Britain circa 1910.

In current-U.S.-dollar terms-the preferred measure of the unipolar-dominance school-the United States has already fallen far from being a global hegemon and unipolarity itself is waning, since China will soon have as much economic potential to balance the United States as did the Soviet Union during the cold war.

At the beginning of the 1990s, the United States was indeed not only stronger than any other state individually, but its power relative to even the collective power of all other major states combined grew from 1990 to 2000. Although the growth was small, America almost reached the crucial threshold of 50 percent of major-power product necessary to become a true global hegemon. So it is understandable that we were lulled into a sense of security, believing we could do as we wished, whenever and wherever we wished. The instability and danger of the cold war quickly became a distant memory.

Near the time of the Iraq War, it would have required virtually every major power to actively oppose the United States in order to assemble a counterbalancing coalition that could approximate America's potential power. Under the circumstances, hard, military balancing against the United States was not a serious possibility. So, it is not surprising that major powers opted for soft-balancing measures-relying on institutional, economic and diplomatic tools to oppose American military power. And yet we are beginning to see "the conflict of history" repeat itself.

Even with less relative power, in the run-up to the Iraq War, people grossly underrated the ability of Germany, France, Russia and China, along with important regional powers like Turkey, to soft balance against the United States; for instance, to use the United Nations to delay, complicate and ultimately deny the use of one-third of U.S. combat power (the Fourth Infantry Division) in the opening months of the Iraq War. This is not yet great-power war of the kind seen in centuries past, but it harkens the instability that future unilateral efforts may trigger.

The balance of world power circa 2008 and 2013 shows a disturbing trend. True, the United States remains stronger than any other state individually, but its power to stand up to the collective opposition of other major powers is falling precipitously. Though these worlds depict potential power, not active counterbalancing coalitions, and this type of alliance may never form, nonetheless, American relative power is declining to the point where even subsets of major powers acting in concert could produce sufficient military power to stand a reasonable chance of successfully opposing American military policies.

Indeed, if present trends continue to 2013 and beyond, China and Russia, along with any one of the other major powers, would have sufficient economic capacity to mount military opposition at least as serious as did the Soviet Union during the cold war. And it is worth remembering that the Soviet Union never had more than about half the world product of the United States, which China alone is likely to reach in the coming decade. The faults in the arguments of the unipolar-dominance school are being brought into sharp relief. The world is slowly coming into balance. Whether or not this will be another period of great-power transition coupled with an increasing risk of war will largely depend on how America can navigate its decline. Policy makers must act responsibly in this new era or risk international opposition that poses far greater costs and far greater dangers.

A COHERENT grand strategy seeks to balance a state's economic resources and its foreign-policy commitments and to sustain that balance over time. For America, a coherent grand strategy also calls for rectifying the current imbalance between our means and our ends, adopting policies that enhance the former and modify the latter.

Clearly, the United States is not the first great power to suffer long-term decline-we should learn from history. Great powers in decline seem to almost instinctively spend more on military forces in order to shore up their disintegrating strategic positions, and some like Germany go even further, shoring up their security by adopting preventive military strategies, beyond defensive alliances, to actively stop a rising competitor from becoming dominant.

For declining great powers, the allure of preventive war-or lesser measures to "merely" firmly contain a rising power-has a more compelling logic than many might assume. Since Thucydides, scholars of international politics have famously argued that a declining hegemon and rising challenger must necessarily face such intense security competition that hegemonic war to retain dominance over the international system is almost a foregone conclusion. Robert Gilpin, one of the deans of realism who taught for decades at Princeton, believed that "the first and most attractive response to a society's decline is to eliminate the source of the problem . . . [by] what we shall call a hegemonic war."

Yet, waging war just to keep another state down has turned out to be one of the great losing strategies in history. The Napoleonic Wars, the Austro-Prussian War, the Franco-Prussian War, German aggression in World War I, and German and Japanese aggression in World War II were all driven by declining powers seeking to use war to improve their future security. All lost control of events they thought they could control. All suffered ugly defeats. All were worse-off than had they not attacked.

As China rises, America must avoid this great-power trap. It would be easy to think that greater American military efforts could offset the consequences of China's increasing power and possibly even lead to the formation of a multilateral strategy to contain China in the future. Indeed, when China's economic star began to rise in the 1990s, numerous voices called for precisely this, noting that on current trajectories China would overtake the United States as the world's leading economic power by 2050.⁸ Now, as that date draws nearer-indeed, current-dollar calculations put the crossover point closer to 2040-and with Beijing evermore dependent on imported oil for continued economic growth, one might think the case for actively containing China is all the stronger.

Absent provocative military adventures by Beijing, however, U.S. military efforts to contain the rising power are most likely doomed to failure. China's growth turns mainly on domestic issues-such as shifting the workforce from rural to urban areas-that are beyond the ability of outside powers to significantly influence. Although China's growth also depends on external sources of oil, there is no way to exploit this vulnerability short of obviously hostile alliances (with India, Indonesia, Taiwan and Japan) and clearly aggressive military measures (controlling the sea-lanes from the Persian Gulf to Asia) that together could deny oil to China. Any efforts along these lines would likely backfire-and only exacerbate America's problems, increasing the risk of counterbalancing.

Even more insidious is the risk of overstretch. This self-reinforcing spiral escalates current spending to maintain increasingly costly military commitments, crowding out productive investment for future growth.

Today, the cold-war framework of significant troop deployments to Europe, Asia and the Persian Gulf is coming unglued. We cannot afford to keep our previous promises. With American forces bogged down in Iraq and Afghanistan and mounting troubles in Iran and Pakistan, the United States has all but gutted its military commitments to Europe, reducing our troop levels far below the one hundred thousand of the 1990s. Nearly half have been shifted to Iraq and elsewhere. Little wonder that Russia found an opportunity to demonstrate the hollowness of the Bush administration's plan for expanding NATO to Russia's borders by scoring a quick and decisive military victory over Georgia that America was helpless to prevent. If a large-scale conventional war between China and Taiwan broke out in the near future, one must wonder whether America would significantly shift air and naval power away from its ongoing wars in the Middle East in order to live up to its global commitments. If the United States could not readily manage wars in Iraq and Afghanistan at the same time, could it really wage a protracted struggle in Asia as well? And as the gap between America's productive resources and global commitments grows, why will others pass up opportunities to take advantage of America's overstretched grand strategy?