No simple answer. Many books have been written about the subject. There were various codes (not just one) in use by the Germans in WW2. The Germans had different codes for the German Foreign Service, Military Intelligence, Navy, Army, Air Force, etc. Some were broken easily, until the Germans changed them. The British, French or Americans would crack them (entirely or a portion) for a short time until the next change. It was a process that never ended and continued the entire pre-war and war period. The most important German Code was code-named by the Allies: "Ultra". It was broken over a period of years. This is probably 'the code' that you have heard about. Google Ultrafor more details.
A top secret code-breaking group called "Ultra"
The British code breaking group in Bletchley Park did.
Unrestricted submarine warfare (in the Atlantic).
Diesel-Electric submarines had been in use for years before WWI; there were many commissioned boats on both sides of the war before it even started, so there is no one particular submarine used in WWI. However, the one boat that arguably had the most impact at the beginning of WWI was the German submarine U-20, which was responsible for sinking the British passenger liner RMS Lusitania. This single submarine attack had repercussions throughout the war, causing policy shifts in Germany considering submarine warfare, and which affected submarine warfare in general forever.
Submarine warfare is a military tactic used by many nations where submarines are used to attack enemy ships or as a direct deterrent. This was used extensively by the Germans during World War II.
A U-boat was a submarine used by Germany in World War 1 and World War 2.
Trench Warfare.
German submarine warfare
Underseaboat.
U-boats
In WW1, the German policy of unrestricted submarine warfare irritated neutral nations such as the United States and eventually helped public opinion to support the US entry into the war. In WW2 the German policy of unrestricted submarine warfare in the Atlantic irritated nominally neutral nations such as the United States and provided a rationale for increasing US support of Britain and its allies. In WW2 the American policy of unrestricted submarine warfare in the Pacific probably contributed to the degradation of Japanese economic capabilities as the war progressed.
Germany's submarine warfare was unrestricted. These attacks led to World War I.
F.-O Busch has written: 'Krieg der \\' -- subject(s): German Naval operations, Naval operations, Naval operations, German, Submarine, Submarine warfare, U 110 (Submarine), World War, 1939-1945
Unrestricted submarine warfare (in the Atlantic).
unrestricted submarine warfare
Germany's unrestricted submarine warfare
Most likely Germany, they had the U-boats (underwater boats)
"Unrestricted Submarine Warfare" was the policy followed by the German Navy to have German submarines without warning sink ships that were transporting food and other resources. U-boat was a term in WW1 referring to a submarine.
Unrestricted German submarine warfare was destroying US ships and killing US passengers. The Americans were also angry at the Zimmerman Telegram, in which Germany asked Mexico to attack the US.