What is the relationship between interstate wars and the probability of civil war onset? We argue that defeated interstate wars of higher conflict intensity are more likely to result in civil war onsets than victorious wars of lower intensity. Using the Doyle and Sambanis dataset (2000) for civil wars and the Correlates of War dataset (2010) for interstate wars, our logistic analyses yield evidence that interstate wars resulting in higher battle deaths are related to higher likelihood of civil war while war outcome is not as significant. Such results indicate that interstate war damage is more decisive in determining civil war onset than its outcome. The main significance of this research lies in that by analyzing interstate wars according to their different intensity levels and outcome, it offers a more thorough investigation into the external factors that affect civil war onset. Moreover, it casts doubt on previous rallying effect studies which generally see external conflicts as opportunities to consolidate state power and challenges conventional arguments regarding war outcome and leader tenure by implying that Pyrrhic victories do little to deter civil war outbreaks.