In the last chapter we saw that the concept of aggression was incorporated inconsistently into the Treaty of Versailles, revealing tensions between Wilson’s collective security model and more conventional approaches based on the narrow self-interest of the major powers in the aftermath of devastating conflict. In this chapter it will be further demonstrated how the latter tendency eroded to a significant extent the League system, as some states sought to clarify the meaning and consequences of League obligations, while others circumvented or undermined League procedures as situations affecting international security arose. Nevertheless, on certain occasions the moral duty contained in article 10 of the Covenant was invoked and conflicts were contained. Thus, although the concept of aggression failed to prevent the outbreak of World War II, it did contribute to the resolution of small-scale disputes.