Here is the opinion in Wilderness Society v. USFS.

And the tribal amicus brief: Tribal Amicus Brief

An excerpt:

Today we revisit our so-called  “federal defendant” rule, which categorically prohibits private parties and state and local governments from intervening of right on the merits of claims brought under the National Environmental Policy Act of 1969 (“NEPA”), 42 U.S.C. §§ 4321  et seq. Because the rule is at odds with the text of Federal Rule of Civil Procedure 24(a)(2) and the standards we apply in all other intervention of right cases, we abandon it here. When construing motions to intervene of right under Rule 24(a)(2), courts need no longer apply a categorical prohibition on intervention on the merits, or liability phase, of NEPA actions. To determine whether a putative intervenor demonstrates the “significantly protectable” interest necessary for intervention of right in a NEPA action, the operative inquiry should be, as in all cases, whether  “the interest is protectable under some law,” and whether “there is a relationship between the legally protected interest and the claims at issue.” Sierra Club v. EPA, 995 F.2d 1478, 1484 (9th Cir. 1993). Since the district court applied the “federal defendant” rule to prohibit intervention of right on the merits in this NEPA case, we reverse and remand so that it may reconsider the putative intervenors’ motion to intervene.