With the end of the Cold War, a popular parlor game in foreign ministries, think tanks, and academia has been to develop a theory of international relations that best explains the new international order. Although there is widespread agreement that the United States is the world's most powerful country in military, economic, and diplomatic terms, and is likely to remain so for the foreseeable future, there is little agreement as to how the rest of the world will react to America's lead. Concepts such as balancing, bandwagoning, buck-passing, and free riding, to name just a few, have been advanced and debated. And although none presents a unified field theory, each explains some aspect of international relations. Theory has an even more difficult time explaining the relationship between the United States and the United Kingdom (UK), especially its remarkable endurance over the past 6 decades. The U.S.-UK partnership flourished during World War II, deepened during the long twilight struggle...