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How much trade did the US conduct with Germany durring WWII? How much US-German trade was there in WWI for that matter?
It fell off, there was a effort to circumvent the blockade via neutral nations, like Norway, Netherlands, Belgium, Spain, Italy, ect... That dwindled as German cash and credit became scarce, the British Navigation Certification system took hold, and the neutrals were occupied by Germany.
... Also US neutrality laws of the 1930s prohibited trade with belligerent nations, and I doubt FDR was going to make an exception for Spain (especially given that he'd made exceptions for the British and French)....
Congress rewrote the Nuetrality Acts in late 1939 to allow doing business with belligerent nations. This was expressed as the "Cash and Carry" policy. Germany was legally free to purchase as much war material as anyone in the US. That was necessary to get approval from pro Germans, Anglophobes, and other opposition to altering the Nuetrality Acts. Roosevelt, the Warhawks, interventionists, & others were comfortable with this as they knew: 1. nazi Germany lacked cash & credit for its war economy. 2. the Allied blockade would further restrict imports from the US. 3. Buercratic interdiction of German exports occurred, delay in export documents ect...
Ending the previous Neutrality Acts had a favorable effect on the US economy. France and Britain were cash flush and had perfect credit in 1939 & immediately placed large orders for war materials and opened negotiation for afar more. This massive influx of cash and credit lines lifted the US out of the lingering Depression.