Diplomacy is an instrument of strategy that great powers use to survive and gain an advantage in competition with other powerful states. Excellence in diplomacy is a vital prerequisite to the success and endurance of great powers. Diplomatic skills atrophied in the United States after the end of the Cold War, as we came to rely on military technology and economic sanctions as the main tools of our foreign policy. But now we are entering a dangerous age in which great powers are competing for the things they have competed over from the beginning of time: territory, resources, influence, and...