Congress enacted the Wild Free-Roaming Horses and Burros Act (the Act), 6 U.S.C. §§ 1331-1340, to protect all unbranded and unclaimed horses and burros on federal land from capture, branding, harassment, and death. The Act authorized the secretary of the interior and the secretary of agriculture to enter into agreements with state and local governments in furtherance of the Act’s purposes. The secretaries executed an agreement with the New Mexico Livestock Board (the Board) (plaintiff) in 1973; however, the Board then terminated the agreement on the ground that Congress lacked the power to regulate horses and burros on public lands unless the horses and burros moved in interstate commerce or were damaging public lands. After the Board rounded up and sold 19 unbranded and unclaimed burros at auction, the Bureau of Land Management, under Secretary of the Interior Thomas Kleppe (defendant), demanded that the State of New Mexico (plaintiff) recover the burros and return them to federal lands. The plaintiffs filed a suit in district court, alleging that the Act was unconstitutional. The district court agreed with the plaintiffs and enjoined Kleppe from enforcing the Act. Kleppe appealed. The United States Supreme Court granted review.