The U.S. and China are already the two most powerful countries in the world. As allies, they would be unstoppable. Is the era of a double superpower taking shape?

When China sneezes, the whole world gets a cold. Bill Clinton recognized this during his term as United States president, speaking of the "potential challenge that a strong China could present to the United States in the future." At the same time, he warned of the risk presented by a "weak China," which could destabilize large regions of Asia.

Now Clinton's successor and fellow Democrat Barack Obama is looking for ways to work more closely with the giant nation, with its 1.3 billion people. President Obama believes that cooperation with China is essential in the coming years. "The major challenges of the 21st century, from climate change to nuclear proliferation to economic recovery, are challenges that touch both our nations, and challenges that neither of our nations can solve by acting alone," the U.S. president said during his recent visit to China.

In China, meanwhile, politicians, economists and the military come to much the same conclusion when they brood over how best to interact with that old superpower, the U.S. "In the 21st century," says the Chinese president and party leader, Hu Jintao, "relations between China and the US are among the most important in the world." There is the perception that, without American help, it will take a long time for China to achieve the "moderate prosperity for all citizens" that the Communist Party promises its people and uses to justify its rule.

Never have the two countries been more dependent on one another than today. Without the American market and American investments, things wouldn't be looking as good as they do in China. But at the same time, many Americans would be struggling to make ends meet during the current economic downturn without cheap imported Chinese goods. And the American government would no longer be able to function if China's central bank didn't buy so much American debt. Last year, China held U.S. Treasury securities worth around $800 billion (€570 billion).