Harold Laski argued for international functionalism from his distinctive socialist perspective. He opposed the existing international system based on the principle of state sovereignty. He also criticised the international federalism proposed as an alternative to the existing system. Although Laski began to devise and present his functionalist case in the 1920s, the circumstances of the following decade led him to adopt and adapt some Marxist ideas and to place less emphasis on functionalism. During and after the Second World War he reconsidered the possibilities for international functional organisation. Although fragmented and undeveloped, his functionalist theory was innovative. By the end of the 1940s he had expressed it in a variety of publications as he reflected on the international conditions of that decade. Unlike what is probably the most well-known functionalist case of the early to mid-twentieth century – that of David Mitrany – Laski's argument bears affinities with the later neofunctionalist theorists. Laski's functionalism was underpinned by the critique of sovereignty which made his political philosophy distinctive. Reasons can be detected for the changes in his attitude to and emphasis on functionalism.