Today in 1945, the Commander-in-Chief of the German Navy, Admiral Karl Donitz, ordered all of his nation's U-boats to cease offensive operations and return home. Thus ended the Battle of the Atlantic, a struggle that ran the entire length of the Second World War and saw the sinking of thousands of ships and the loss of tens of thousands of lives.
The German U-boat, short for Unterseeboot, made its initial appearance during the First World War. They proved to be highly effective weapons, sinking millions of tons worth of cargo vessels carrying goods from North America to England and France. In fact, U-boat activity was one of the reasons the United States entered the war in 1917. So successful was the U-boat campaign that the Treat of Versailles, which ended the war, forbade the building of submarines by Germany. After Adolf Hitler came to power in 1933, the German Navy began skirting this prohibition. When the Second World War began in September, 1939, Germany had a small but highly effective submarine force consisting of 57 U-boats.
Admiral Donitz's opinion of how submarines should be used was controversial among his peers. Most nation's navies saw submarines as more or less reconnaissance and defense vessels that should be assigned to fleets the same way other escort ships were. Donitz believed in unrestricted submarine warfare similar to what had been practiced during part of the First World War. His plan called for sinking merchant ships using a tactic developed a generation earlier---the wolf pack. Better radios and the Enigma code machine allowed the U-boats to communicate with each other while on patrol. This allowed the subs to attack a convoy from various positions at various times, just like a pack of wolves.
The first two and a half years of the war saw the U-boats become a major threat to the lifeline that kept England in the war. Each month, hundreds of thousands of tons of shipping went to the bottom of the Atlantic. The U-boats were aided in their hunt by German surface raiders, fast battlecruisers that, in the beginning, were considered more dangerous than submarines. At first, the Royal Navy and a few Free French warships stood alone against the menace. But in April, 1941, US President Franklin Roosevelt extended what he called the 'Pan-American Security Zone' all the way to Iceland. Despite American neutrality, US Navy warships began escorting convoys to a point in the mid-Atlantic, where they were handed off to the Royal Navy. Even though no declaration of war existed, the United States was at war on the high seas against Germany months before Pearl Harbor.
Germany declared war against the US on December 11, 1941. By the second week of January, 1942, U-boats were appearing off the eastern seaboard. Over the next few months, their attacks proved nearly cataclysmic. The US Navy was woefully unprepared for anti-submarine warfare and lacked enough destroyers and coastal vessels to protect the thousands of ships making their way up and down the coast. As a result, people living in many towns along the Atlantic coast saw something that few thought possible: American merchant vessels on fire and sinking within sight of land. Some U-boat commanders were so bold during this time that they pursued their prey into water too shallow to dive. At least one sub was spotted in Galveston Bay, Texas and more than one crew saw the bright lights of Manhattan from New York Harbor. It took the United States months to absorb the hard-learned lessons of their British allies.
Technology and increased ship production eventually began to take its toll on the U-boat fleet. Improved sonar, radar and code-breaking meant that the once nearly-invisible subs had very few places to hide. Escort carriers and increased numbers of destroyer escorts and frigates gave rise to hunter-killer groups whose only mission was to find and sink U-boats. Once found, the subs often found themselves the victims of a hedgehog attack, named after the 24-barraled mortar that fired a pattern of direct contact bombs. If one of the bombs made contact with a sub, it's detonation would cause the other 23 mortars to also detonate, creating a massive explosion that was almost guaranteed to end the sub's life.
By middle of 1943, the U-boats were losing the Battle of the Atlantic. While Allied merchant ship losses were still high and would remain so for some time, new ships were finally being built at a faster rate than the sinkings. Heavy bombers could now cover the entire North Atlantic, robbing the U-boats of an aircraft-free zone in the middle of the ocean. The Allied air forces also began regular patrols of the Bay of Biscay, the only route to the Atlantic for subs based in occupied France. So many U-boats were lost in the Bay that the German crews began calling the area "The Valley of Death."
Advancing Allied technology forced the Germans to push the limits of submarine technology. By the end of the war, the Type XXI boat had been introduced. It's design and performance was so revolutionary that had it been put into service two or three years earlier, the outcome of the Battle of the Atlantic may have been completely different. Several of these boats ended up in the Soviet Union after the war and served as the design basis for the Zulu and Whiskey-class submarines. The Type XXI hull design also influenced the USS Nautilus and the Albacore-class submarines of the US Navy.
In the end, the German U-boat fleet failed to cut off the line of supply from North America to Britain. In all, the Allies lost over 3,500 merchant vessels, while the Germans saw 783 of their subs sank out of a total of a little more than one thousand built. Nearly 31,000 merchant sailors died in the battle, while over 28,000 submariners died. After Admiral Donitz issued his stand-down order on May 4, 1945, most of the submarines headed for Germany. After the war in Europe ended on May 8th, 154 U-boat commanders surrendered their ships. Of those, 121 were scuttled in deep water off Northern Ireland and Scotland, where they remain today.