The United Steelworkers of America (plaintiff) and American Manufacturing Company (defendant) had a collective-bargaining agreement (CBA) that required arbitration of all disputes over “the meaning, interpretation, and application” of its provisions. A no-strike clause prohibited strikes with no exceptions, unless the company refused to comply with the arbitrator’s decision. An injured employee who left work accepted a partial disability settlement but wanted to return to his job. The union brought a grievance, claiming that a seniority provision in the CBA meant the employee was entitled to return. When the company refused to arbitrate the grievance, the union sued in federal court. The court found the worker could not claim seniority or employment rights after accepting the disability settlement. The appellate court affirmed, finding the grievance frivolous and thus not subject to arbitration. The Supreme Court granted review.