Besides infringement and preliminary ruling procedures, actions for annulment form a third important category of procedures brought before the Court of Justice of the European Union (CJEU). A subset of these actions is ‘horizontal litigation’, where EU institutions litigate against other EU institutions. Based on a new dataset covering all horizontal annulment conflicts (1957–2017), this contribution analyzes conflict patterns. I identify the most typical litigant constellations and link them to substantial battles over competences including winners and losers. Using the example of EU external affairs, I show how annulment actions have shaped the relationship between EU institutions over time, with impact as significant as treaty changes. In sum, the analysis of this so far ‘forgotten’ type of procedure furthers our systematic understanding of policy development and competence allocation in the EU system.