Scholars have long debated the causes of late third party state joining in ongoing inter-state wars. This research has generally concluding that peace-time conditions, measured in terms of: third party capabilities; proximity to warring states; and inter-state alliances, are determining factors in the decision to join. However, these studies utilize theories derived from static pre-war measures of capabilities and motivation to explain late joining; indeed, the same measures that fail to predict participation at war's outset. Further, extant research has no explanation for why weak and non-proximate states every participate. Existing theory thus fails to provide a comprehensive explanation of joining behavior. This project contends that a resolution lies the interaction between pre-war conditions and intra-war events. Intra-war events that are allowed to vary on a per battle basis, including change in combat location and alliance entry and exit from combat, reveal new information about the war and its progress, thereby forcing third party states to recalculate their initial decision to abstain in relation to their pre-existing situation. Incorporation of intra-war processes helps to better explain decisions by third party states to join ongoing inter-state wars late in their development, and why states that frequently choose to abstain (e.g., weak states) ever choose to participate. This project is executed using a combination of ex post facto historical case studies, a theory of joining based on pre and intra-war environments, and large-N empirical analysis on all inter-state wars 1823-1988, conducted utilizing a novel collection of event-level data based on inter-state war battles.