Emma Mcilroy’s apparel company, Wildfang, had been working overtime to move its production out of China in the months since President Trump launched his trade war. Finding another factory that could produce jumpsuits and button-downs — while every other U.S. importer was also jockeying for space — was a huge drain on the 11-person staff’s attention.

But when Ms. Mcilroy saw on Wednesday night that a federal court had ruled most of Mr. Trump’s tariffs illegal, all of that work paused. Would the tariffs be gone when her next shipment arrived in August, or not?

“I have absolutely no idea where it’s going. I am learning in real time how to run my business,” Ms. Mcilroy said Thursday. “Yesterday I would’ve told you, ‘Yes, absolutely, you’re going to see me manufacturing stuff in Vietnam.’ Today I’m not sure.”

American businesses are rapidly digesting the latest twist in Mr. Trump’s roller-coaster trade war, which has made it impossible to plan more than a few weeks in advance. It’s particularly hard on industries that place their orders entire seasons ahead of time.

The details of Wednesday’s decision seemed likely to bring relief. A three-judge panel of the U.S. Court of International Trade ruled that the Trump administration had acted illegally in using an emergency powers law to impose 30 percent tariffs on goods from China, 25 percent tariffs on most goods from Mexico and Canada, and 10 percent on everyone else. The court gave the White House 10 days to halt the new duties.

Hours later, a higher court stayed the decision.

If the initial ruling sticks, it will preclude the return of steeper “reciprocal” tariffs that Mr. Trump paused for 90 days in early April. It might even allow companies that have paid the emergency tariffs over the past several months to claim refunds, already an established process at Customs and Border Protection.

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