To some extent, it is possible to argue that the sinking of the Reuben James had little effect on U.S. foreign policy with respect to World War II. There was no immediate reaction to the sinking in terms of Congressional action or military action on the part of the US.
However, it can be argued that the sinking of the Reuben James contributed to a decrease in isolationism in the US. It was not long after the sinking of the ship that Congress repealed many of the provisions of the Neutrality Acts that had been passed in the 1930s. By repealing these provisions, Congress was moving the US towards a more active role in the war.
One can argue, then, that the sinking of the Reuben James helped push the US towards a less isolationist stance. It should be noted, however, that America was still officially neutral towards Germany even after the sinking and did not actually go to war with Germany until Germany declared war on the US after the Pearl Harbor attack.