First, the Church teaches—and many Catholic natural law scholars hold—that the emergence of a “world community” as a distinct political community necessitates a corresponding “authority” (the precise parameters of which the Church has carefully avoided specifying). The second reason might be described as practical. Whether we like it or not, the argument goes, the UN exists. Hence it’s better for the Holy See to be involved, if only to help derail some truly bad ideas.
The following seems to be written by someone who is more neocon than conservative (see his comments regarding Iran) -- Humility and Hubris by William Kilpatrick.