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12 ask the United States to fulfill part of that duty by assessing what water rights it holds for them. The government owes the Tribe at least that much.

Begin with the governing legal principles. Under our Constitution, “all Treaties made” are “the supreme Law of the Land.” Art. VI, cl. 2. Congress can pass laws to implement those treaties, see, e.g., Bond v. United States, 572 U. S. 844, 851, 855 (2014), and the Executive Branch can act in accordance with them, see, e.g., Fok Yung Yo v. United States, 185 U. S. 296, 303 (1902). But the Judiciary also has an important role to play. The Constitution extends “[t]he judicial Power” to cases “arising under … Treaties made, or which shall be made.” Art. III, §2, cl. 1. As a result, this Court has recognized that Tribes may sue to enforce rights found in treaties. See Moe v. Confederated Salish and Kootenai Tribes of Flathead Reservation, 425 U. S. 463, 472–477 (1976). Other branches share the same understanding. In enacting the Indian Trust Asset Reform Act of 2016, Congress confirmed its belief that “commitments made through written treaties” with the Tribes “established enduring and enforceable Federal obligations” to them. 25 U. S. C. §5601(4)–(5) (emphasis added). The Executive Branch has likewise and repeatedly advanced the position—including in this very litigation—that “a treaty can be the basis of a breach-of-trust claim” enforceable in federal court. Brief for Federal Parties 22–23, n. 5.

What rights does a treaty secure? A treaty is “essentially a contract between two sovereign nations.” Washington v. Washington State Commercial Passenger Fishing Vessel Assn., 443 U. S. 658, 675 (1979). So a treaty’s interpretation, like “a contract’s interpretation, [is] a matter of determining the parties’ intent.” BG Group plc v. Republic of Argentina, 572 U. S. 25, 37 (2014). That means courts must look to the “shared expectations of the contracting parties.”