Interventionism in the sense which the Centre studies it is a post-Cold War phenomenon. To be sure, there were military interventions before then but it is only in the period after 1989 - 1991 (during which the Berlin Wall fell and the Soviet Union collapsed) that legal instruments came into force (principally United Nations Security Council Resolutions) which allegedly provide a legal basis for interventionism.
This legal basis is thin. The legal documents listed here (click on "Legal documents" in the menu on the left hand side of this page) fall very far short of the thick layers of treaty law, costumary law and rulings by the International Court of Justice which form the legal basis for non-interventionism. Those legal documents can be seen under the menu tab "Non-interventionism".
None of the legal instruments listed in this section have explicitly superseded the Charter of the United Nations or the rulings of the International Court of Justice which contradict them. Interventionists of course want to expand their legal base. No interventionist, however, has ever proposed drawing up a general right of interventionism which would legalise, for instance, a Chinese intervention in the internal affairs of the United Kingdom. Instead, interventionism, such as it is practised today, is exclusively by powerful Western states against weaker, non-Western ones. NATO's operation against Libya is only the latest and most obvious example of this trend.