A rarely explored subset of territorial disputes is that of domestic-level territorial disputes. In such a dispute, a substate group disagrees with the country's central government over whether the government should have sovereign control over the entire territory of the state; that is, the dissatisfied group is demanding independence. If the independence-minded group prevails, new boundaries and states are created through a territorial change known as secession. This study explores secession's track record (i.e., how often secessions have been successful in resolving the underlying territorial disputes) and reasons why some secessions are followed by militarized conflict over the new boundary. The results—based on an examination of the aftermath of all twentieth century secessions and the application of more sensitive criteria than those used in prior research—reveal that while most secessions leave unresolved territorial disputes in their wake, these disputes are not particularly prone to escalate into militarized confrontations. Moreover, intangibly (i.e., ethnically) based disagreements over the new boundary play a much greater role in militarized conflict onset than do tangibly (i.e., economically or strategically) based territorial disputes. Finally, contrary to expectations from the ethnic secession literature, peaceful secessions significantly decrease the likelihood that the new boundary will be contested militarily.