Liberal internationalism, as a constellation of ideas, ideologies, and movements, came to the fore in Europe and the United States in the late nineteenth century, where it flourished until the mid-1940s. Although as an overt ideology it diminished greatly during the Cold War years, liberal internationalism nevertheless remains embedded in international institutions and in parts of the international system. This chapter explores liberal internationalism in both its ideational and institutional forms, focusing particularly on its prescriptions for peace and its connections and overlaps with pacifism. It highlights proposals for an international police force as a contentious issue between muscular and pacifist strands of liberal internationalism.