The United Mine Workers of America, District 17 (defendant), represented employees of Eastern Associated Coal Corporation (plaintiff). The parties’ collective-bargaining agreement (CBA) required Eastern to prove just cause to discharge an employee in arbitration. Otherwise, the arbitrator would order reinstatement. James Smith worked on Eastern’s road crew, a job that required driving heavy equipment and subjected him to Department of Transportation (DOT) regulations and random drug testing. When Smith tested positive for marijuana, Eastern tried to discharge him. The arbitrator found no just cause for discharge and ordered Smith reinstated conditioned on a 30-day suspension, substance-abuse treatment, and further drug testing. When Smith again tested positive for marijuana, Eastern again tried to discharge him. But the arbitrator found mitigating circumstances warranted reinstatement, including Smith’s 17-year history as a good employee and his credibly testifying that personal family problems caused a one-time relapse. The arbitrator ordered Smith reinstated conditioned on a three-month suspension, paying arbitration costs, continued substance-abuse treatment, and random drug testing. The arbitrator also required Smith to give Eastern a signed, undated resignation letter effective immediately if he failed another drug test within five years. Eastern sued in federal court, claiming the award contravened public policy. The court concluded Smith’s reinstatement did not violate public policy, and the appellate court affirmed. The Supreme Court granted review.