In order to expand its production of zinc concentrate in northern Alaska, Cominco, Inc., needed to increase its on-site electrical generating capacity at its Red Dog Mine (the Mine) facility. The expanded zinc production at the Mine would add more than 40 tons of nitrogen oxide per year to the air emissions at the site. The expansion plans triggered a Prevention of Significant Deterioration (PSD) requirement pursuant to the Clean Air Act (CAA). Under the PSD provision, no major air pollutant emitting facility may be constructed unless the facility is equipped with “the best available control technology” (BACT). The Mine submitted its expansion proposal to the state permitting authority, the Alaska Department of Environmental Conservation (ADEC) (plaintiff), for review and approval. ADEC determined that BACT for the Mine was selective catalytic reduction (SCR) which reduced nitrogen oxide emissions by 90 percent and which was technologically, economically, and environmentally feasible for the Mine to implement. The Mine countered and offered to install new equipment which would reduce emissions only by 30 percent. The ADEC relented and issued a permit allowing construction of a facility to begin. The state agency also subsequently modified its position that SCR was economically infeasible for the Mine to implement. The Environmental Protection Agency (EPA) (defendant) intervened and issued an order preventing the Mine from proceeding with construction of the facility. The ADEC petitioned the court of appeals to review the EPA order. The appellate court held EPA possessed the authority under the CAA to issue an order to halt construction of the facility. The U.S. Supreme Court granted certiorari to review.