Border guards of the inner German border
The United States Army maintained a substantial and continuous military presence at the inner German border throughout the entire period from 1945 to after the end of the Cold War. Regular American soldiers manned the border from the end of the war until they were replaced in 1946 by the United States Constabulary, a lightly armed constabulary force responsible for border security. It was disbanded in 1952 after policing duties were transferred to the German authorities. In its place, two dedicated armoured cavalry regiments were assigned to provide a permanent border defence. The 2nd Armored Cavalry Regiment based at Nuremberg and the 14th Armored Cavalry Regiment based at Fulda– later replaced by the 11th Armored Cavalry Regiment – were tasked with monitoring the border using observation posts, ground and air patrols, countering border intrusions and gathering intelligence on Warsaw Pact activities. Unlike their East German counterparts, U.S. soldiers did not stay for more than 30 days on the border, though they carried out regular patrols around the clock using foot and helicopter patrols. They also used a variety of technical measures such as ground surveillance radars to monitor Warsaw Pact troop movements across the border. A rapid reaction force was on constant duty further behind the border to provide backup in an emergency. The American presence on the border provoked political controversy in Germany. During the 1960s the state of Hesse refused to grant U.S. forces land rights to its observation points or allow them to install paved access roads, electricity or telephone lines. It took the view that since there was no legally recognised border, there was no legal reason for their military observation posts to be built along it. By the 1980s the American border presence had become the target of peace activists, who in 1984 blockaded the U.S. Observation Post Alpha with a human chain. The U.S. withdrew from the inner German border in 1991.
U-boat
During World War II, U-boat warfare was the major component of the Battle of the Atlantic, which began in 1939 and ended with Germany's surrender in 1945. The Armistice of November 11th, 1918 ending World War I had scuttled most of the old Imperial German Navy and the subsequent Treaty of Versailles 
of 1919 limited the surface navy of Germany's new Weimar Republic to only six battleships (of less than 10,000 tons each), six cruisers, and 12 destroyers. To compensate, Germany's new navy, the Kriegsmarine, developed the largest submarine fleet going into World War II. British Prime Minister Winston Churchill later wrote "The only thing that really frightened me during the war was the U-boat peril."
In the early stages of the war the U-boats were extremely effective in destroying Allied shipping due to the large gap in mid-Atlantic air cover. Cross-Atlantic trade in war supplies and food was extensive and critical for Britain's survival. The continuous action surrounding British shipping became known as the Battle of the Atlantic, as the British developed technical defences such as ASDIC and radar, and the German U-boats responded by hunting in what were called "wolfpacks" where multiple submarines would stay close together, making it easier for them to sink a specific target. Britain's vulnerable shipping situation existed until 1942, when the tides changed as the U.S. merchant marine and Navy entered the war, drastically increasing the amount of tonnage of supplies sent across the Atlantic. The combination of increased tonnage and increased naval protection of shipping convoys made it much more difficult for U-boats to make a significant dent in British shipping. Once the United States entered the war, U-boats ranged from the Atlantic coast of the United States and Canada to the Gulf of Mexico, and from the Arctic to the west and southern African coasts and even as far east as Penang. The U.S. military engaged in various tactics against German incursions in the Americas; these included military surveillance of foreign nations in Latin America, particularly in the Caribbean, to deter any local governments from supplying German U-boats.