What 1945 United States law made state regulation of insurance the general rule?

The 1945 United States law that made state regulation of insurance the general rule is the McCarran–Ferguson Act.

Congress enacted the McCarran–Ferguson Act in response to uncertainty about the federal government’s authority over insurance, including the effect of the Supreme Court’s 1944 decision in United States v. South-Eastern Underwriters Association. The law declared that state regulation and taxation of insurance were in the public interest.

The act generally preserved state authority while allowing federal laws to apply when they specifically concern insurance. It also provided a limited exemption from federal antitrust laws for certain insurance activities, provided those activities were regulated by state law. That exemption is narrower than a blanket exclusion from federal oversight.

The act did not create a single national insurance regulator. Instead, insurance remains principally regulated by the states, with each state setting rules for many insurers, products, licensing requirements, and market practices.