Although evidence shows that territorial disputes fundamentally shape relations among states, we know surprisingly little about when territorial claims are made. We argue that revisionist states have incentives to make territorial claims when the great powers that manage the system are in crisis. We identify five main sources of systemic instability and develop measures of each of them, demonstrating that the majority of territorial claims in Europe are drawn at times when regional great powers are embroiled in crisis, for example, 1848 or 1870 during the nineteenth century. The claims that emerge at these times are not necessarily among states involved in the crises that generated turmoil (e.g., Prussia and France in 1870). We use a newly developed spatial measure of historical boundary precedents in Europe from 1650 to 1790 to demonstrate that the effect of this known spatial correlate of where claims are drawn matters only when the European system is in crisis. We further demonstrate that this claim-timing pattern is general to the global system of states. In the appendix we corroborate our explanation of our findings with a detailed case study of the territorial claims that led to the contemporary Italian state's formation.