The National Environmental Policy Act (NEPA) generally requires a federal agency to prepare an Environmental Impact Statement (EIS) for a proposed 'major Federal action.' The Federal Motor Carrier Safety Administration (FMCSA) is an agency within the Department of Transportation. It is responsible for motor carrier safety and registration. FMCSA has no statutory authority to impose or enforce emissions controls nor to establish environmental requirements unrelated to motor carrier safety. The President of the United States, under the North American Free Trade Agreement, had expressed an intention to lift the moratorium on Mexican domiciled motor carriers to operate in the United States. FMCSA published for comment proposed rules concerning safety regulation of Mexican motor carriers. Congress then enacted §350 of the Department of Transportation and Related Agencies Appropriations Act, which provided that no funds appropriated could be obligated or expended to review or to process any application by a Mexican-domiciled motor carrier for authority to operate in the interior of the United States until the FMCSA implemented specific application and safety-monitoring requirements for such carriers. Acting pursuant to NEPA's mandates, FMCSA issued a programmatic EA for the proposed Application and Safety Monitoring Rules. The EA considered the environmental impact in the categories of traffic and congestion, public safety and health, air quality, noise, socioeconomic factors, and environmental justice. Vital to the EA's analysis, however, was the assumption that there would be no change in trade volume between the United States and Mexico due to the issuance of the regulations. The EA concluded that the effects were minor and could be addressed and avoided in the inspections process itself. The EA (environmental assessment) concluded that the issuance of the proposed regulations would have no significant impact on the environment, and hence FMCSA, on the same day as it released the EA, issued a FONSI. In November 2002, the President lifted the moratorium on qualified Mexican motor carriers. Public (P) had already filed petitions for judicial review of the Application and Safety Monitoring Rules, arguing that the rules were promulgated in violation of NEPA and the CAA (Clean Air Act). The Court of Appeals agreed with Ps, granted the petitions, and set aside the rules. The appeals court reasoned that the EA was deficient because it failed to give adequate consideration to the overall environmental impact of lifting the moratorium on the cross-border operation of Mexican motor carriers. FMCSA was required to consider the environmental effects of the entry of Mexican trucks because 'the President's rescission of the moratorium was 'reasonably foreseeable' at the time the EA was prepared. The Court of Appeals directed FMCSA to prepare a full CAA conformity determination for the challenged regulations. The Supreme Court granted certiorari.