This article is concerned with the historical trajectory and legacy of British liberal internationalist ideas in the opening three decades of the twentieth century. Despite this body of ideas being a major force behind the establishment of International Relations (IR) in Britain following the Great War, only scant attention is paid to its pre-war configuration. The article attempts to remedy this gap by focusing on internationalist thought prior to and during the war. It is argued that internationalist ideas during the Great War accelerated a drift towards institutional arguments, which are herein distinguished from moral arguments, and that the concept of anarchy played a major role in this shift in internationalist ideas. While the transformation of liberal internationalist ideas during the war constitutes a central backdrop to the early practices of British IR, it should not overshadow the powerful, underlying continuity in ethico-political convictions entertained by internationalists before and after the Great War.