>Anyway, what regulation is responsible for Walmart and Amazon putting local retailers out of business?

Walmart + Amazon combined are only ~16% of the retail business. They're not monopolies. The fact they put a small minority of businesses out of business does not mean they're a monopoly. This is likely part efficiencies and also part regulatory capture via the insane zoning/building regulations in this country and tax breaks that can favor large corporations.

> After a big merger put AT&T in charge of the majority of telephone lines in the US, the company used its control over infrastructure to drive its competitors out of business and increase its portfolio. The Justice Department tried to break up AT&T but failed; it was in the settlement of this case that AT&T was first federally regulated in 1913. Yes, AT&T's monopoly grew between 1913 and 1982, but your causality is backwards. They regulated it because it was already a monopoly.

... It was not already a monopoly. Hundreds of phone companies emerged and by shortly before your noted date of "regulation" those competitors held the majority market share. It became a "monopoly" after the government literally quasi-nationalized them (AT&T) to the point the fucking Postmaster General was basically in charge of it, they became intertwined with regulators, and then the drive for "universal telephone service" and regulatory commissions ensured the regulatory compliance pushed exactly into AT&Ts business model. AT&T intertwined lawmakers even brought in economics quacks to talk up natural monopolies to argue for policies to create the regulations that made AT&T a monopoly. So you have it backwards -- the regulated it from a minority market holder to an unnatural monopoly and lawmakers created this monopoly under the auspices they essentially needed to legislate a "natural" (misnomer) monopoly into existence.