When Jim McNerney, Boeing’s chief executive, wrote an e-mail to his employees this week, he offered them a bleak vision of what could happen if the aircraft-maker caved in to the demands of striking machine workers.

“US auto companies ... all but fatally wounded themselves years ago by promising unsustainable wage and benefit levels and by agreeing to contract conditions (including job guarantees) that limited their flexibility to run their businesses in the face of intense global competition,” Mr McNerney warned. “Today, their market shares continue to fall, and their lay-offs have grown by the thousands.”