U.S. Vice President Joe Biden concluded a brief three-country tour of Asia that took him to China, Mongolia and Japan. While there is always some trepidation when Mr. Biden travels — while he is a genuine foreign policy expert, he has a tendency to make off-the-cuff remarks that get him in trouble — the trip was a success. He achieved his key objectives: Working on building a relationship with his counterpart, Chinese Vice President Xi Jinping, the man tapped to succeed President Hu Jintao next year, and reassuring China and other Asian nations, especially Japan, of the ongoing U.S. commitment to the region.

The U.S.-China relationship is one of the most important in the world. For better or for worse, the two countries need to cooperate to tackle key international problems; bad relations between them will undermine regional peace and security. Engagement is required across all levels of government and society, but leadership and direction from the very top is essential. Mr. Biden's trip followed up on a visit to the U.S. in January by Mr. Hu, which set the tone for a cooperative and positive relationship.

But with Mr. Hu scheduled to step down next year, U.S. officials are trying to get the measure of the man who will succeed him. And while the broad outlines of Mr. Xi's career are known — party functionary from a ranking family, a victim of the Cultural Revolution who has seen poverty firsthand, and who has since spent time in some of the most dynamic parts of China — he remains an unknown quantity (not unlike many of the country's second tier of top leaders).