When Standard Oil was broken up, it wasn't a 100% monopoly. Nor was AT&T. I'm not sure what your criteria are, but if you're just being pedantic, it doesn't see

When Standard Oil was broken up, it wasn't a 100% monopoly. Nor was AT&T. I'm not sure what your criteria are, but if you're just being pedantic, it doesn't seem to have any historical bearing. The great monopolies of US history that prompted legal change never had complete control. They just had de facto control due to outsized influence and predatory (or at least anticompetitive) behavior.