What makes some wars longer and more severe than others is an important question in international relations scholarship. One underexplored answer to this question is the role that third party joiners play in lengthening conflicts, especially those states that intervene militarily after a war’s initial stages. This article argues that late joining complicates bargaining by adding new issues to the war and increases uncertainty about the relative balance of forces. Thus, more information will be needed to resolve the bargaining impasse. This means additional fighting and a longer war. This lengthening in turn increases the number of casualties. This is a distinct process from simply having more participants in a war from the outset as those participants would not add uncertainty in the same way that late joiners do, as questions about how those participants affect the relative balances of forces would be answered just as quickly as if there were only two participants at the outset. These claims are supported by a non-proportional hazards model regression, a Cox proportional hazards regression, and an ordinary least squares regression using the Correlates of War interstate war dataset.