The United Steelworkers of America (plaintiff) represented employees of Enterprise Wheel & Car Corporation (Enterprise) (defendant). The parties’ collective-bargaining agreement (CBA) required arbitration of any differences “as to the meaning and application” of the agreement and said the arbitrator’s decision “shall be final and binding on the parties.” The CBA also said that if the arbitrator found that Enterprise had unjustly suspended or discharged an employee, then Enterprise “shall reinstate the employee and pay full compensation . . . for the time lost.” When a group of employees left work to protest Enterprise’s discharging an employee, the union immediately told them to return. But the next day, the company said the workers no longer had jobs “until this thing was settled.” The union filed a grievance, but Enterprise refused to arbitrate. The union brought a specific-enforcement action, and the district court compelled arbitration. The arbitrator found the facts warranted no more than a 10-day suspension. Meanwhile, the CBA expired, but the arbitrator found the reinstatement provisions unconditionally obligated Enterprise and awarded reinstatement with backpay less 10 days’ pay. The district court ordered Enterprise to comply, but the appellate court found the award as to backpay and reinstatement after the CBA expired unenforceable. In addition, the appellate court found the award of backpay due before expiration ambiguous and directed the parties to complete arbitration to calculate those amounts. The Supreme Court granted review.