What is the name of the federal law that created the state-based system of insurance regulation in the U.S.?

The federal law that preserved the state-based system of U.S. insurance regulation is the McCarran–Ferguson Act.

Congress passed the act in 1945 after the Supreme Court’s 1944 decision in United States v. South-Eastern Underwriters Association held that insurance could qualify as interstate commerce subject to federal authority. State regulators and insurers feared that ruling might disrupt the regulatory structure already built by individual states.

McCarran–Ferguson does not create a single national insurance regulator, nor does it require states to regulate every aspect of insurance. Instead, it says that federal laws generally should not invalidate, impair, or supersede state insurance laws unless the federal law specifically concerns the business of insurance. It also gives insurance a limited exemption from federal antitrust laws when state regulation applies.

The act is often mistaken for Gramm–Leach–Bliley or Dodd–Frank. Those laws affect financial services and insurance in particular ways, but neither established the foundational state-centered framework. The National Association of Insurance Commissioners later helped states coordinate through model laws and accreditation standards.