<p>According to contemporary just war theory waging a just war requires that a belligerent fulfill a number of stringent conditions such as just cause, proportionality, and necessity. Failing to fulfill any of them would render a war <i>un</i>just. It is therefore surprising that there has been almost no philosophical work on wars that are unjust from <i>both</i> sides. The fact that mutually unjust wars exist empirically seems fairly obvious. However, we argue that there are two difficult theoretical puzzles at the core of this type of situation that have remained underexplored. This article sorts out these puzzles and advances the view that mutually unjust wars are not only conceptually coherent, but that they constitute a philosophically challenging and practically pressing starting point for thinking on the ethics of war.</p>