Answer:
Great Britain
Step-by-step explanation:
At the beginning of the Great War, the United States were trading with Great Britain. Eventually the United States said that they wouldn't help any belligerent in the war, but still somewhat discreetly traded with Great Britain and gave them weapons. It wasn't until I believe the Zimmerman Telegram that was supposedly sent from Germany to Mexico where Germany was asking Mexico to go to war with the United States to reclaim their lost land and stall any United States advance into Europe, that the United States got involved in the war. The Zimmerman Telegram is a widely disputed issue in the realm of WW1. As for your original question though, I believe the correct answer is they were trading profitably with Great Britain. Hope this helped!
-Kirito