Why do adversaries sometimes cooperate to restrain their military competition? Why do they design arms control agreements with intrusive verification in some cases but rely on minimal transparency in others? Amidst ongoing international competition, arms control remains rare despite potential mutual benefits, and agreements vary dramatically in their approaches to monitoring. This book reveals how uncertainty from domestic political changes-such as leadership transitions or social unrest- can enable arms control. It identifies two paths to agreement: during periods of uncertainty, states that previously relied on informal understandings hedge by establishing lightly-monitored agreements, while those that anticipated deception take calculated risks through agreements with intensive verification. Through comprehensive data analysis and rich case studies, Jane Vaynman challenges conventional wisdom about uncertainty in international relations while offering insights for policymakers. As states confront challenges from nuclear competition to emerging technologies, understanding when arms control becomes viable is more vital than ever.