In “Nuclear Beliefs: A Leader-Focused Theory of Counter-Proliferation,” Rachel Whitlark advances a new framework to explain why military force is rarely employed to prevent the spread of nuclear weapons. According to power transition theory, a nuclear weapons program should spark an intense security dilemma with a high risk of war as other nations consider using force to forestall an adverse shift in the balance of power. Contrary to this conventional wisdom, Whitlark demonstrates that even a looming proliferation threat does not pressure all leaders to “think and act similarly” by mulling over preventive war (550). Instead, the article shows that presidents and prime ministers come into political office with prior beliefs about the consequences of proliferation and stability of nuclear deterrence. For some leaders, a sanguine judgement about the ability to manage nuclear-armed adversaries becomes an anchor against even deliberating coercive counter-proliferation strategies. Whitlark marshals archival evidence to show how President John F. Kennedy’s entrenched pessimism about proliferation led him to consider a range of military options against China’s emerging nuclear program. In stark contrast, President Lyndon B. Johnson’s optimism seemed to result in these same options being taken off the table on the eve of the first Chinese nuclear weapons test.