one-worlder

[ wuhn-wurl-der ]
/ ˈwʌnˈwɜrl dər /

noun

a person who supports or believes in any of various movements to establish a world government or a federation of nations stronger than any individual nation, for the purpose of promoting the common good.
a person who believes in the possibility of peaceful cooperation among nations.

Origin of one-worlder

1945–50; one world + -er1
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019