When and where did the Battle of the Atlantic start?
When can be generally accepted as the Invasion of Poland at the beigning of WWII in Europe, and the where can be just as simplistic: "The Atlantic".
Germany's U-Boat fleet was already on wartime patrol and when the war began, they were prepared to pounce. When the war in Europe ended, U-Boats were at sea, maintaing their patrols. Allied Naval and air forces were still searching them out, and they were among the last combatants to call it quits.