  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.
