The well-known British cultural historian and philosopher of history Arnold Toynbee (1889-1975) made indirect references to a possible United Mankind Organisation in his writings. For example, in his book "Mankind an Mother Earth":
"The present independent regional states are incapable of maintaining peace, protecting the biosphere from human pollution, or conserving their irreplaceable resources. This political anarchy can no longer be allowed to continue in an ecumene that has long since become one in the technical and economic fields. What has been needed for five thousand years - and has proved feasible in technology for a hundred years - is a world-wide political organisation consisting of individual cells on the scale of neolithic village communities - so small and manageable that each member knows the other and yet is a citizen of the world state."
See my article on Substack:
Arnold Toynbee and the unity of mankind