THE PERSON WHO holds the office of president of the United States possesses the most influential voice in world affairs today. This is partly because the United States is the recognized global military superpower and the president, as commander in chief, has immediate control of the U.S. military. The president's international influence also derives from the status of the United States as the world's richest and most industrialized nation. Trade with and/or financial aid from the United States can mean the difference between prosperity and economic hard times for many nations. Thus, how the president conducts foreign affairs directly affects people not only in the United States, but around the world as well.

This has not always been the case. In the first few decades of its existence, the United States was a relatively...