I believe during ww2 since before the start they were neutral and it was tought that there was nothing bad with trading with Germany but and I quote "As Second World War began in September 1939, the U.S. was officially neutral until December 1941 when it declared war on Germany. Franklin Roosevelt's foreign policy strongly favored Britain (and France) over Germany in 1939–41" so from this I am led to believe that shortly after the start of WW2 the USA broke trade with Germany