CAMBRIDGE, Mass. - What's the BIG question? If you were trying to predict the course and character of world politics for the next 50 years or so, what question would you like to know the answer to today?

It's easy to think of different candidates, such as: 1) will climate change continue unabated with far-reaching effects on the world economy and low-lying areas?; 2) will the euro survive? 3) will terrorists ever acquire and use a weapon of mass destruction?; 4) will there be a major global pandemic? And so forth.

All good questions, but the future of Sino-American relations should be on everyone's list of Top 5 "Big Questions." And on that subject, the main issue is whether China will continue to tolerate America's extensive and powerful military presence in East Asia or whether it will conduct a sustained effort to drive a wedge between the United States and its current allies and eventually force the United States out of the region.

The current situation is clearly anomalous. Historically, it is somewhat unusual for one great power to have a tight set of alliances in the immediate neighborhood of another great power and to maintain a lot of military force in its vicinity, without the other power having a compensating presence in close proximity to its rival.

To be sure, America's Cold War alliances and military deployments had this same quality -- the United States had large forces in Europe and Asia while the USSR had a very modest presence in the Western Hemisphere -- but this situation mostly reflected the Soviet Union's unfavorable geographical location and relative economic weakness. Moscow would have loved to have gotten the United States out of Asia and Western Europe and pinned it down in the Western Hemisphere; it just didn't have any good way to accomplish any of those goals.

As my sometime co-author John Mearsheimer has repeatedly noted, a rising China is likely to want to force the United States out of Asia. I mean, seriously: What great power would want to be ringed by neighbors who have close security partnerships with its main peer competitor and would want that same rival to keep a lot of potent military forces near its shores?

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The United States certainly didn't like the idea of a large-scale European great-power presence in the Western Hemisphere -- remember the Monroe Doctrine? -- and once it became a great power it lost little time in pushing Britain and France out of its "backyard." It certainly helped that the European powers were always more worried about each other, but the key point is that U.S. leaders understood that the nation's security would be maximized if it were the sole great power in the region. There is no reason to think that Beijing sees this issue any differently.

It is hard to overstate the long-term implications of this issue. If the United States is able to maintain the status quo in Asia and help prevent China from dominating the region, then Beijing will have to focus a lot of attention on local issues, and its capacity to shape politics in other parts of the world will be constrained. By contrast, if China eventually pushes the United States out of Asia, it will have the same sort of hegemonic position in its region that the United States has long enjoyed near its own shores. That favorable position is what allows Washington to wander all over the world telling others what it thinks they should do, and regional hegemony would give Beijing the option of doing the same if it wished. It might even start forging closer ties -- including security ties -- with countries in the Western Hemisphere. That's why the question of how long Beijing will tolerate the U.S. presence in Asia is so important.

So there you have it: One of the Big Questions that will shape the next few decades.

Of course, it is not obvious that China's rise will continue (at least not at the same pace as before); even if it does, its efforts to reduce America's presence in Asia may not succeed. A lot depends on how America's current (and potential) allies respond and also on whether the U.S. national security establishment can reach an enduring consensus on the priority to be given to Asian security in general and China in particular.

That consensus seemed to be emerging during President Barack Obama's first term, but the energy behind the "pivot" seems to have dissipated as the administration has re-engaged in the Middle East maelstrom. No doubt Vice President Joe Biden will try to straighten things out during his visit to the region this week, but long-term success will depend on a lot more than the occasional top-level fly-in.

Stephen M. Walt is a professor of international relations at Harvard University.