As fabric spooled out of a bank of knitting machines in a flare of blue-and-white stripes, workers at the Saint James clothing factory in France’s Normandy region stacked them into piles and cut along patterns to make the company’s iconic Breton sailor’s shirt, worn by celebrities and adored by fans worldwide.

Luc Lesénécal, the company’s chief executive, surveyed an enormous workroom splashed with a rainbow of yarns. Seamstresses had recently put the finishing touches on 50,000 striped shirts and sweaters to fill orders for American stores like Nordstrom and J. Crew.

But his plans to ship for the fall retail season have been thwarted by President Trump’s up-and-down tariff threats. Instead of loading the merchandise into cargo planes, Mr. Lesénécal has parked his entire U.S. export in the company’s warehouse, where it will sit until Wednesday, the deadline Mr. Trump has set for Europe to come up with a deal or face