China will almost certainly pass the United States in the total size of its economy within a decade or so. But if one looks also at military and "soft power" resources, the United States is likely to remain more powerful than China for at least the next few decades. Does it matter?

When nations worry too much about power transitions, their leaders may overreact or follow strategies that are dangerous. As Thucydides described it, the Peloponnesian War - in which the Greek city-state system tore itself apart - was caused by the rise in the power of Athens and the fear that created in Sparta. Similarly, World War I, which destroyed the centrality of the European state system in the world, is often said to have been caused by the rise in power of Germany and the fear that created in Britain. (Though the causes of both wars were also much more complex.)

Some analysts predict that a similar scenario will be the story of power in the 21st century: The rise of China will create fear in the United States, which will lead to a great conflict. But that is bad history. By 1900, Germany had already passed Britain in industrial strength. In other words, the United States has more time to deal with China's growing power than Britain had to deal with Germany's, and the United States does not have to be as fearful. If it were to be too fearful, both sides might overreact. The Chinese, thinking America was in decline, would push too hard, and Americans, worrying about the rise of China, would go too far.

The best way to avoid that is by having a very clear-eyed view of all dimensions of power and how they are changing. The recent Sunnylands summit between President Barack Obama and Chinese President Xi Jinping was a step in this direction.

Another reason it is important not to be too fearful is the diffusion of power. China and the United States - as well as Europe, Japan and other nations - will be facing new transnational challenges on issues such as climate change, terrorism, cyber security and pandemics. These issues, which will only become more urgent, will require cooperation, including help in many cases from nongovernmental agencies.

Obama's 2010 National Security Strategy referred to the fact that the United States has to think of power as positive-sum, not just zero-sum. In other words, there may be times when it is good for the United States (and the world) if Chinese power increases.

Take, for example, China's power to control and reduce its greenhouse gas emissions, the one area where China is undoubtedly a superpower. We should be eager to see China increase its capacities here, including the development of its shale gas reserves. This is a win-win.

In meeting many of the new transnational challenges, the United States has to get away from thinking just about power over others and think about power with others. We do not want to become so fearful that we are not able to find ways to cooperate with China.

World politics today is different from that of the last two centuries. It is now like a three-dimensional chess game in which interstate military power is highly concentrated in the United States, interstate economic power is distributed in a multipolar manner and power over transnational issues such as climate change, terrorism and pandemics is highly diffused. The structure of power is not unipolar, multipolar or chaotic; it is all three at the same time. Thus, a smart strategy must handle different distributions of power in different domains and understand the trade-offs among them.

It makes no more sense to see the world through a purely realist lens - that focuses only on the top chessboard and predicts conflict with China - than it does to see through a liberal lens that looks primarily at a single board and predicts only cooperation. In a tri-level game, a player who focuses on only one board is bound to lose in the long run. Fortunately, China and the United States have more to gain from the cooperation dimension of their relationship than from the conflict one. Both just need to recognize that.

Joseph S. Nye Jr. is a professor at Harvard University and the author, most recently, of "Presidential Leadership and the Creation of the American Era." He wrote this for the Los Angeles Times.

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