[Snowyman]

The first U-boat sunk by the US was March 1, 1942 , by a Hudson Bomber. Another was sunk on March 15, again by a Hudson. The USS Roper managed to get one April 14th.

All well after war was declared.

[Wyrd bið ful aræd]

On or around Sept 11th, 1941, FDR ordered Americans ships to attack German U-boats on sight. Nazi submarines' "very presence in any waters which America deems vital to its defense constitutes an attack. In the waters which we deem necessary for our defense, American naval vessels and American planes will no longer wait until Axis submarines lurking under the water, or Axis raiders on the surface of the sea, strike their deadly blow—first."

There had been multiple incidents before this where US and German vessels had fired on each other, and in which US vessels had assisted British vessels in sinking German Uboats.

http://millercenter.org/president/speeches/speech-3323