Both a declaration of war or actual hostilities would amount to a violation of article 2 (4) of the UN Charter. However, it is not inconceivable that a declaration of war could be a proportionate measure of self-defence and thus compatible with the UN Charter. This is confirmed by references to a “state of war” in state practice, international instruments, and municipal legislation. Yet, it is undeniable that not all armed conflicts constitute war. Hence the question arises whether there is any need to distinguish between “wars” and “conflicts not amounting to wars”. This paper argues that the concept of war has not been made redundant by the UN Charter regime on the use of force. Not only has the state of war not become legally irrelevant to international law by virtue of article 2 (4) of the UN Charter, but, quite on the contrary, the state of war has become “more relevant than ever”. The actions bringing about a state of war are now illegal whereas before the adoption of article 2 (4) they laid within the discretion of each state. At most, the Charter regime has rendered as obsolete the notion that states are free to decide when to engage in war.