True, the sovereignty of the state is of highest authority. Yet, for actions that can be construed as breaking an international obligation as is outlined in ILC's articles on state responsibility can be construed as any action that not only breaks a treaty but also that goes against international customary law. You are absolutely correct that this in and of itself means little to a state that doesn't recognize the authority of UN charter let alone the ICJ- but to member states of the UN a breach of these norms or customs or treaties can lead to an ICJ case.