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“This case was never about the union or NLRB telling Boeing where it could put its plant,” Mr. Solomon explained. “This was a question, for us, of retaliation.”

Boeing and the machinists union historically have had a testy relationship, including five strikes since 1977. Both sides said they hoped a new contract agreed to this week would bring in an era of labor peace for one of the country’s biggest exporters.

“For the next five years, I take the parties at their words that they’re really going to work on a better relationship,” Mr. Solomon said. “I think there’s no reason to not be optimistic.”