The United States (plaintiff) acquired two parcels of land in Louisiana to add to a wildlife refuge pursuant to the federal Migratory Bird Conservation Act. One parcel was purchased in 1937, and the other was acquired through condemnation in 1939. In each acquisition, the Little Lake Misere Land Company (Little Lake) (defendant) reserved production rights for oil, gas, and other minerals for 10 years and thereafter until Little Lake ceased mineral production on the parcels. The purchase agreement and condemnation order both specified that, after 10 years or thereafter when production ceased, the mineral rights terminated, and the United States would take fee simple title to the parcels. Louisiana then enacted Act 315 of 1940, which directed that mineral rights reservations in effect when the United States acquired lands could not be terminated. Little Lake never started any mineral production on either parcel. In 1955, the United States leased gas and oil production rights on both parcels. Little Lake, however, believed it maintained the mineral rights pursuant to Act 315 and entered into various contracts to transfer these rights. The United States brought a quiet title action in 1969 in the United States District Court for the Western District of Louisiana, claiming it obtained fee simple title to the parcels in 1947 and 1949, respectively, pursuant to the written instruments. The district court granted summary judgment for Little Lake, and the court of appeals affirmed on the basis that state law governs real estate issues, regardless of federal involvement. The United States Supreme Court granted certiorari.