At the latest since the Second Iraq War, the «just war theory» has again been on all minds, which since late antiquity was intended to distinguish legitimate from illegitimate acts of war. However, this theory has always been suspected to have been used more as a fig leaf than as a hindrance to military operations. Based on historical examples, the article traces how the doctrine was developed and – especially in view of the Spanish conquest wars in Central America – how it was used to assess the legitimacy of wars. It is shown that its potential was ambivalent indeed; but similar to military interventions which sometimes prevent greater suffering, the just war theory should be regarded as a potentially helpful (though not infallible) theoretical framework.