From the draft Global Trends 2030 report:
Will the United States be able to work with new partners to reinvent the international system, carving out new roles in an expanded world order? How the US evolves over the next 15-20 years—a big uncertainty—will be among the most important variables in shaping the international order.
The United States’ relative economic decline vis-a-vis the rising states is inevitable and already occurring, but Washington’s future role in the international system is much harder to assess. An economically restored US—the likeliest scenario—would be a “plus” in terms of the capability of the international system to deal with major global challenges during the long transitional period to a fully multipolar world, although a strong US would not alone guarantee that the growing global challenges were met. A weak and defensive US, on the other hand, would increase the chances of a dysfunctional international system.
Even as the United States’ economic weight is overtaken by China—perhaps as early as the 2020s based on several forecasts—the US most likely will remain primus inter pares among the other great powers in 2030 because of its preeminence across a range of power dimensions and legacies of its leadership role. Nevertheless, with the rapid rise of multiple other powers, the “unipolar moment” is over and Pax Americana—the era of unrivalled American ascendancy in international politics that began in 1945—is fast winding down.