Here is the opinion.

From Justice Gorsuch’s opinion:

Like some courts before it, the Washington Supreme Court read Yakima as distinguishing in rem from in personam lawsuits and “establish[ing] the princi­ple that . . . courts have subject matter jurisdiction over in rem proceedings in certain situations where claims of sovereign immunity are asserted.” 187 Wash. 2d, at 868, 389 P. 3d, at 574.

That was error. Yakima did not address the scope of tribal sovereign immunity. Instead, it involved only a much more prosaic question of statutory interpretation concerning the Indian General Allotment Act of 1887. See 24 Stat. 388.

***

We leave it to the Washington Supreme Court to address these arguments in the first instance. Although we have discretion to affirm on any ground supported by the law and the record that will not expand the relief granted below, Thigpen v. Roberts, 468 U. S. 27, 30 (1984), in this case we think restraint is the best use of discretion. Determining the limits on the sovereign immunity held by Indian tribes is a grave question; the answer will affect all tribes, not just the one before us; and the alternative argument for affirmance did not emerge until late in this case. In fact, it appeared only when the United States filed an amicus brief in this case—after briefing on certiorari, after the Tribe filed its opening brief, and after the Tribe’s other amici had their say. This Court has often declined to take a “first view” of questions that make their appearance in this posture, and we think that course the wise one today. Cutter v. Wilkinson, 544 U. S. 709, 718, n. 7 (2005).

Background materials here.