How do we explain great power behavior towards emerging great powers? Great powers can either try to stop the emergence of new great powers or they can try to help their rise, either passively by doing nothing to stop them or by actively helping them gain power. Which approach does a great power choose in any given situation and why does it do so? Surprisingly, no theory has directly addressed this question. The goal of this dissertation is to refine existing theories to answer this question, both by providing explanations of past great power behavior as well as making predictions of how the United States will react in different situations in the future when it faces emerging great powers. I approach this puzzle using a framework that focuses on threat and means. I argue that it is only when both the threat and means are high that a great power will try to stop an emerging great power. When the threat is low, the existing great powers will usually help the rise of the emerging great power in an attempt to accomplish other goals. When the threat is high but the means are low, the great power is forced to appease the emerging great power until it can change its means from low to high. Different theoretical approaches to international relations argue that threats and means are measured by different variables. In this dissertation I develop how realism and ideological approaches predict states will measure threats and means. I show that my version of realism is a powerful explanation of great power behavior, and ideology has less explanatory power, by using congruence testing, process tracing, and cross-case comparison on three sets of historical cases: (1) the great powers' reaction to Prussia during the wars of German unification; (2) the reaction of the Soviet Union and the United States to China during the Cold War; and (3) the reaction of the United States to a growing China in the post-Cold War era.