The Denver Building & Construction Trades Council (defendant) and its affiliated craft unions had a long-standing dispute with a general contractor who used nonunion workers. When the contractor hired a nonunion electrical subcontractor for an otherwise all-union jobsite, a single union picket patrolling for two weeks managed to keep every worker except electrical off the jobsite. The contractor relented and ordered the electrical subcontractor off the job, terminating the subcontract. The electrical subcontractor filed charges with the National Labor Relations Board (NLRB) (plaintiff), claiming the unions induced an unlawful strike to force the general contractor to cease doing business with it. The NLRB found the unions engaged in a prohibited secondary boycott and entered a cease-and-desist order. But the appellate court found the boycott primary and refused to enforce the NLRB order. The Supreme Court granted review.