In Asia, the Chinese army was overwhelmed, and in Europe, the Germans were caught up in resistance to Britain, France, and the United States.

At the outbreak of the war, there were about 3,000 ocean-going merchant ships flying the British flag and 1,000 offshore trade vessels regularly plying the British coastal waters, with a total tonnage of 17.5 million tons. In mainland waters and in the North Atlantic, these merchant ships are usually organized into convoys and escorted by 4-6 armed vessels armed with depth charges. After receiving information that German warships might be on the route, the British Home Fleet would also send heavy warships to escort ships over long distances.

The lifeline of maritime transport is crucial. Without sea transportation, Britain could not even solve the problem of feeding the people, let alone manufacturing weapons, and it can be said that once there was no goods to import, Britain was not even qualified to enter the war.

However, the German navy was not ready to take advantage of this weakness of the British, and the German navy was scheduled to be involved in the war in the mid-40s of the 20th century. Unable to engage the British fleet face to face, Germany resorted to attacking merchant ships.

The "Deutschland" class heavy cruisers, commonly known as "pocket battleships", were the first class of German main battleships, with guns up to 28 cm caliber. Designed to attack commercial vessels only, the ships of this class are diesel powered and have excellent endurance. Before the outbreak of war, the German Navy "Admiral Schell" and "Count Spee" were sent to the Atlantic, ready to go into battle immediately after the declaration of war. Both ships frequently attacked unescorted merchant ships sailing in the Central and South Atlantic, and both were partially successful. "Admiral Scheer" sank 16 ships with a total tonnage of 100,000 tons during the five-month raid.

However, pocket battleships were all solitary and vulnerable to concentrated fire from the powerful British Royal Navy. In December 1939, while operating in the South Atlantic, the "Earl Spee" was spotted by a squadron of cruisers of the British Royal Navy, and was eventually forced to sink itself at the mouth of the Pratt River. The new heavy warships of the German Navy were not very suitable for breaking the engagement. The Hippert-class heavy cruisers used high-pressure steam thrust, which was extremely fuel-intensive, thus limiting their range. Nevertheless, the "Admiral Hippel" once managed to intercept a fleet off the coast of West Africa, sinking 7 ships with a total tonnage of 27,210 tons.

The battle cruisers "Scharnhorst" and "Gneisenau" sank the Royal Navy aircraft carrier "Glorious" in the Norwegian campaign, but because their firepower was too light, they were not really battleships, and their role in the later battles can be said to be insignificant, as the Royal Navy sent the old "R" and "Queen Elizabeth" class battleships on escort duty, and the two ships suffered a crushing defeat in the North Atlantic attack on the British fleet.

The German Navy also made extensive use of armed merchant cruisers, with good results. These ships resemble ordinary merchant ships in design, but under their camouflage they hide heavy weapons. Judging by the number of ships they sank, the armed merchant cruisers did not achieve much success, and they never posed a threat to the Allied supremacy. However, the attack of armed merchant cruisers caused great panic among merchant ships, forcing the Royal Navy to send a large number of troops to pursue them.

Although the German propaganda machine continued to trumpet the Navy's surface attacks as typical material, the German Navy was unable to concentrate its large warships on a decisive blow to the Allies, and the scattered attempts to break diplomatic relations were at best harassing to the Allies.

In contrast to the surface fleet, the U-boat forces of the German Navy posed an extremely serious threat to the Allies.

Commander of the submarine forces of the German Navy, Karl Brown. Admiral Dönitz took submarines very seriously. He believed that to complete the naval blockade of Britain, a fleet of about 250 or 300 submarines was needed. According to Hitler's pre-war "Plan Z", the ambitious Nazi naval construction program, Germany planned to build such a fleet and become combat ready by 1943. But while building a powerful submarine fleet, Germany also plans to build two squadrons of giant battleships and build the world's largest battleship. Due to the preference of the German High Command for battleships, the plan for the construction of submarines was postponed due to the relegation of the German High Command, which, in Dönitz's opinion, was impractical. In his opinion, only by relying on submarines could there be any hope of completing the blockade of Britain. …,

Dönitz's knowledge of submarines stems from the First World War.

The Imperial German Navy was the first to use submarines in large numbers to attack enemy merchant ships, and although submarines were still relatively crude and simple in construction at the time, they were quite successful. Germany's U-boats dealt a huge blow to British trade in the Mediterranean, and the unrestricted submarine warfare of 1917-1918 almost brought Britain to its knees. This was also a major reason for the prohibition of the development of submarine forces in Germany in the Treaty of Versailles signed after the war.

Despite the limitations of the treaty, Germany secretly established submarine design offices in the Netherlands in 1922 and Berlin in 1927!

A few months before Hitler came to power in 1932, the Weimar government approved a plan to build a navy, including 16 U-boats. After the rise of the Nazis, the realization of the naval construction program was accelerated. In 1935, Herman. Goering announced to the world that Germany would re-arm and refused to accept the Treaty of Versailles. Hitler also worked to get Britain to sign a Naval Agreement with Germany, which allowed Germany to have up to 45 percent of the Royal Navy's naval strength. But puzzling is that, despite Britain's painful experience of being attacked by submarines in World War I, the agreement allowed the German submarine force to be in line with Britain in total tonnage.

It was in this year that #Janitz was commissioned to command Germany's first post-war small submarine fleet, and began training a new submarine fleet to accomplish the tasks that the German Navy had narrowly escaped during World War I. After taking charge of the German submarine forces, Dönitz determined the most suitable types of submarines for inshore and ocean-going operations, and the number of submarines to be built.

#阝尼茨所确定的需求之一就是建造一种近海潜艇, as a result, the Type Piper submarine was quickly put into production. Actual combat showed that the Type submarine was agile and dynamic, and could dive rapidly in 25 seconds, and because of its small hull and easy maneuverability, it quickly earned the nickname "canoe".

In the Battle of the Atlantic, Germany used mainly Type Yin submarines.

Like the Type Piper, the Type Yin was originally intended for combat against Finland and was built in 1930. The number of submarines of this type was large, and by the end of the war, more than 800 units were produced. Although Germany intended to use the submarine for ocean-going operations, it was relatively limited in size in order to maximize the production of the submarine within the treaty limits, which in turn enhanced its flexibility and ability to dive quickly.

Like most submarines at the time, the Type Yin was powered by a diesel engine for surface navigation and a battery-powered electric engine for underwater. When diesel-electric driven, the Type Yin submarine can reach a speed of 17.5 knots, which is more than enough to encircle the slow-moving merchant fleet; However, when driving underwater, it can only reach speeds of up to 5 knots and can only travel at this speed for a few hours. TQPE Yin submarines carry 11~14 torpedoes. Early production of this type of submarine was also armed with a deck gun, and later in the war, deck guns were often replaced by more prominent heavy anti-aircraft guns.

The larger Type submarines were designed in accordance with the requirements of ocean-going operations. The early production of Type submarines had a long range and could operate in the southern hemisphere, often cruising in the South Atlantic. Late-production models have a further increased range and can operate in the Indian Ocean and even the Pacific Ocean without the need for refueling. The living conditions of its crew have also been improved, making it suitable for long-term activities, and the number of torpedoes carried is 22, which is about 1.5 times that of the Type Yin submarine. Despite this, the crew still preferred the smaller Type Yin submarine because of its fast diving speed and strong underwater maneuverability. The larger Type submarine takes 40 seconds to make an emergency dive, and it is very vulnerable to air attacks

Until the summer of 1939, Hitler was preaching that the German Navy was not in danger of getting involved in the war. At that time, #阝尼茨属下仅有46艘作战潜艇more than half of them were "landlubbers" that could only sail in the near sea, that is, Type Lutcher-type submarines, which were mainly used for training. …,

In the first eight months of the war, German submarines attacked Britain's main supply lines with very few operations, and almost all operations were directed against individual ships that were out of the fleet. Moreover, during the Norwegian campaign, in order to guarantee combat operations in the North Sea, Germany even withdrew all submarines operating in the Atlantic.

That changed later. During the interwar period, Dönitz developed the tactical theory of boat group attack. After taking charge of Hitler's submarine forces, he experimented with the concept of "wolf pack tactics":

This is a tactical concept of a coordinated attack with a submarine group in the open ocean. Another submarine warfare technique that Dönitz advocated was what he learned in World War I, namely that attacks from the surface at night are best used.

In 1939, he published a book detailing these tactics, including how small-contoured submarines were not easily spotted at night, and how submarines in the thirties of the twentieth century could reach speeds much higher than the average merchant ship when navigating on the surface. Sadly, no one seems to notice this book in the UK!

The initial zone of war extended westward to about 800 kilometers west of Iceland.

In the first months of the war, German submarines had considerable success, mainly attacking merchant ships that sailed alone to Britain. German submarine commanders were more worried about British destroyers, because these ships were equipped with the Assic underwater sonar system, which could detect the distance and bearing of submarines sailing underwater.

However, Britain does not have enough anti-submarine destroyers, and never will have, because Britain is mass-producing small frigates and light cruisers for anti-submarine missions to replace anti-submarine destroyers, and this production plan will take several years to reach large quantities!