Since the UN Charter came into effect in 1945, there have been numerous incidents in which one or more of the five major powers (at least arguably) violated the Charter's Article 2(4) prohibition of force. Such incidents notwithstanding, this book demonstrates how the Charter restrains the major powers' military actions. As an instrument of international order, the Charter provides a framework of legal rules restricting the use of armed force. Although these rules are subject to auto-interpretation by the major powers (as a consequence of their veto), they create an expectation of compliance that subjects the major powers' military actions to international scrutiny. To reduce the likelihood of resistance from states threatened by such actions, major powers exercise prudential restraint, altering the manner and timing of their military actions in accordance with the legal arguments offered to justify those actions as consistent with the Charter and therefore not threatening to the existing international order. The book evaluates the efficacy of the Charter using large-N methods and five case studies: US intervention in the Caribbean, 1953–61; Anglo- French intervention in Egypt, 1956; Soviet intervention in Hungary, 1956; US–British intervention in Iraq, 1990–98; and US–British intervention in Iraq, 1999–2003. The book's extensive focus on the two Iraq cases provides a basis for timely evaluation of the continuing salience and possible reforms of the UN Charter system.