The United Kingdom had always been dependent on the seas for its economy, importing vital food and other supplies for its own survival. Germany knew this well. Too weak to directly challenge the British and French fleets, the German Navy adopted a strategy of using surface ships, submarines, and aircraft to raid Allied commerce shipping.
On 3 Sep 1939, within hours of the British declaration of war on Germany, German submarine U-30 attacked what the submarine captain thought was a British auxiliary cruiser. The ship had turned out to be the passenger liner Athenia, the very type of ship that the German Navy ordered its submarines to avoid. The death of 112 civilians aboard Athenia started what Winston Churchill would christen the Battle of the Atlantic.
The British and the French immediately responded with a blockade on German ports. This interfered with raw materials coming into Germany from Northern Europe, but overall it did not affect the course of the battle much. Meanwhile, the Royal Navy set anti-submarine hunting groups out to sea. On 14 Sep 1939, while one of these hunting groups was out in search of German submarines, U-39 spotted the group first and launched torpedoes at the carrier HMS Ark Royal. The carrier narrowly escaped harm as the torpedoes detonated prematurely. Three days later, HMS Courageous of another hunting group was less fortunate, discovered, and sunk by U-29. While the British considered alternatives in dealing with German submarines, German submarine U-47 scored an even greater victory by sneaking into the British naval base at Scapa Flow, Scotland, United Kingdom, and sank the old battleship HMS Royal Oak at anchor, killing 833. Two weeks later, in an attempt to follow up on the success at Scapa Flow, two torpedoes from U-56 struck HMS Nelson. Her crew was lucky that the torpedoes failed to detonate.
While the Kriegsmarine hunted for British targets, a major strategy change took place in Germany. On 16 Oct 1939, Grand Admiral Erich Raeder announced Adolf Hitler’s orders that “all merchant ships definitely recognized as the enemy can be torpedoed without warning.” It was further explained that all ships, even those flying flags of neutral nations, could be targeted based on the German captain’s discretion if the ships were bound for British ports. “Neutral shipping had been sunk before, inadvertently, or by reckless commanders. Now it was Kriegsmarine policy.”
Beginning in late 1940, German submarines attacked in the Rudel, or wolfpack, in a tactic that German Admiral Karl Dönitz devised before the war. With the Wolfpack tactic, some of the submarines in the Wolfpack would keep the convoy escorts occupied while one or two of the submarines would sneak by into the center of the formation and attack the transports.
Although submarines had scored the first victories in the Battle of the Atlantic, the German submarine fleet was small at the start of the war. There were only 57 in operational status, some of which were in use as minelayers rather than raiders. The responsibility of the early attacks on Allied shipping was shared by surface ships as well. Ships ranging from battleships to armed merchant cruisers were dispatched into the Atlantic Ocean to threaten merchant shipping coming in and out of Britain and France. Although many surface merchant raids were extremely successful, this branch of the German plan of war to blockade Britain was limited by the policy that surface ships should avoid escorted convoys to minimize potential losses.
German naval vessels were in the center of public attention in this opening stage of the battle; nevertheless, naval mines were another principle weapon against Allied shipping. While contact mines were laid just below the waves on the British coast just deep enough for ships to make contact, German magnetic mines were also used, which could be laid in deeper waters, detonating when a ship neared and causing damage with the shock wave of the explosion. On the night of 22 Nov 1939, a German plane was observed dropping something via parachute into the River Thames in southern England, United Kingdom. Upon investigation, the package delivered was a magnetic mine, and it landed in the mud off Shoeburyness instead of the river. Two Royal Navy officers retrieved the mine and turned it to scientists, who reverse-engineered it. The magnetic principles were discovered, and the technique of degaussing, the process of demagnetizing with electric coils, was developed to prevent ships from triggering off enemy magnetic mines when sailing nearby. The Germans also had in their arsenal a pressure-activated naval mine but did not deploy them until it had become apparent that the British had devised methods to defeat the magnetic mines.
Just as the submarines were proving themselves worthy in the Battle of the Atlantic, Grand Admiral Erich Raeder reallocated funding from submarine construction to that of capital ships. Although Dönitz would later reverse this decision, the delay in the expansion of the German submarine fleet gave the United Kingdom the little bit of breathing room that it desperately needed in this early stage of the war.
Although German attacks continued through the winter months of 1939 to 1940, many of the German ports in the Baltic Sea became frozen, thus significantly slowing the German naval efforts. The German campaign in Norway in Apr 1940 also drew away many warships that were on raiding duties at the start of the war. It would not be until after the fall of France in Jun 1940 when the German Navy would escalate the actions in the Atlantic Ocean.