Modular localization and the theory of string-localized fields have revolutionized several key aspects of quantum field theory. They reinforce the contention that local symmetry emerges directly from quantum theory, but global gauge invariance remains in general an unwarranted assumption, to be examined case by case. Armed with those modern tools, we reconsider here the classical Okubo-Marshak argument on the non-existence of a "strong CP problem" in quantum chromodynamics.