This is the third in a series of weekly essays analyzing the issues that the candidates are, or should be, discussing.
For more than four decades, the most important foreign policy challenge facing any President has been managing relations with the other superpower. The Soviet Union is the only state that can threaten America's existence; it is the principal U.S. rival for influence around the world; and its totalitarian political system is anathema to American values.
Those facts of international life remain, but today they do not seem quite the immutable laws of nature they did four years ago, when Americans...