In the time between the beginning of WWII and the US entry into the war in December of 1941, the US provided more and more help to the British in response to U-Boat attacks on merchant shipping.
At first, the US response was limited. The US, for example, agreed to the "Destroyers for Bases" deal with Britain in which the US gave Britain more destroyers to use for anti-submarine warfare. Eventually, however, the US started to actively patrol and to escort convoys on its own even thought this was really the next thing to open participation in the war.
By the summer of 1941, the US Navy was protecting British and American ships from U-Boat attacks by escorting convoys much of the way across the Atlantic.
In these ways, the US responded to U-Boat attacks with greater involvement in the protection of British shipping even though these actions were tantamount to participation in the war.