The Battle of the Atlantic played a very significant part in World War Two. In World War Two, after the escape at Dunkirk and the inspiration of the Battle of Britain, the Battle of the Atlantic was Britain's next nightmare.
As an island Britain needed to bring in a vast amount of food and military equipment to survive the war.
The German submarine force (U-boats) severely damaged our ability to survive the war - hence Churchill’s quote above when he feared we would be starved out of the war.
A great deal of our raw materials came from America and therefore had to cross the Atlantic. In normal times this journey could be hazardous because of the weather but in the war the German submarines lead by Admiral Raeder proved a very real threat. Nazi Germany estimated that they needed to sink 150 merchant ships each month to starve us out.
German submarines hunted in what were called wolf-packs. British supply ships crossed in convoys and the ships that brought in our food etc. were slow and they could barely protect themselves. After leaving America they were reasonably safe while in American water and they were also more safe when they approached British waters as we could give the ships fighter plane cover. It was in the mid-Atlantic that we were at our most vulnerable and where to start with the U-boats could run riot.
German submarines had direct access to the Atlantic once France had fallen in the spring of 1940. Massive submarine pens were built near Bordeaux and the impact they had can be seen from the following figures :