This chapter evaluates the potential for legal regulation of the resort to cyber warfare between states under the 'jus ad bellum'. It first sets out in some detail a debate as to the correct interpretation of Article 2(4) of the United Nations (UN) Charter 1945, with respect to cyber warfare. The chapter argues that 'the Article 2(4) debate' often misses the fact that an act of cyber warfare can be considered a breach of a different legal rule: the principle of non-intervention. It then considers some issues in applying either the prohibition of the use of force or the principle of non-intervention to cyber warfare. The exhaustive and exhausting debate over the applicability of Article 2(4) is the focus of much of the legal literature on cyber warfare. There is need to introduce 'new' law, but to refocus the cyber warfare debate around an existing legal duty.