Final answer:
The case that determined insurance was interstate commerce and subject to federal regulation when conducted across state lines was United States v. South-Eastern Underwriters Association.
Step-by-step explanation:
The court decision that established that insurance was interstate commerce when conducted across state lines and therefore subject to federal regulation was United States v. South-Eastern Underwriters Association. This case is not explicitly mentioned in the provided excerpts. However, it is a significant case regarding the interpretation of the commerce clause and federal regulatory power. Unlike in Gibbons v. Ogden, which dealt with federal authority over navigation and interstate commerce, the South-Eastern Underwriters case specifically addressed the insurance industry. By recognizing insurance transactions crossing state lines as interstate commerce, the Supreme Court confirmed the federal government's power to regulate the insurance business, upholding the spread of federal regulatory regimes and expanding the breadth of the commerce clause once again.