When will US become enemy

[ReM]

I sailed out of St Nazaire on November 16, 1941....after patrolling my assigned grid, AM 41 in foul weather I decided to go to the US coast in order to be the first drum beater after war is declared on december 12 1941.

After lurking off the coast of the US, and waiting for december 12 to arrive I find that the US is still very neutral....after waiting for about a week I finally sink a US tanker on december 18, the symbol for the sinking is green....aaaah a neutral kill...

Sick and tired of neutrals I make my way to the coast of Halifax, Canada...I find an enemy convoy with a Revenge class BB in it....I fire 4 eels and it goes down in a cloud of flames...I sunk Royal Oak

Anyway, does being close to the American coast on december 12 prevent the US from entering the war...or is there something strange going on? I have patrolled the US coast many times after december 12 and it has always been hostile afaik...