In Wilderness Society, the plaintiffs alleged that the Forest Service violated NEPA by failing to prepare an Environmental Impact Statement and consider reasonable alternatives when it adopted a travel plan designating roads and trails in a National Forest for motorized vehicle use. Three groups representing recreation interests moved to intervene but were opposed by the conservationist plaintiffs. The district court applied the "federal defendant" rule and denied intervention of right and permissive intervention. 
Reviewing the case en banc, the Ninth Circuit explained that the rationale for the federal defendant rule was that "NEPA is a procedural statute that binds only the federal government," and the only defendants can be "the governmental bodies charged with compliance." On that basis, an intervenor on the government defendant's side would not have a "significantly protectable" interest. The court found that such rationale "mistakenly focuses on the underlying legal claim instead of the property or transaction that is the subject of the lawsuit". There are possible circumstances where private parties seeking to intervene in NEPA cases can demonstrate an interest 'protectable under some law,' and a relationship between that interest and the claims at issue.
The court declared that the federal defendant rule "eschews practical and equitable considerations and ignores our traditionally liberal policy in favor of intervention". The court further found that the rule is inconsistent with its "approval of intervention of right on the side of the federal defendant in cases asserting violations of environmental statutes other than NEPA," such as the Federal Land Policy and Management Act, the Endangered Species Act, and the Administrative Procedure Act. The Ninth Circuit abandoned the bright-line rule against intervention as defendants in a NEPA action, holding that courts should be permitted to analyze a motion to intervene on a case-by-case basis and examine the factual context under the same standards applicable to all other intervention cases.