Throughout World War II, Britain and the European theatre of war in general was heavily reliant on supplies from the USA, the Empire and elsewhere, which had to be brought in by sea. Initially the Germans' attempts to disrupt these trade routes involved a mixture of submarine (U-boat) attacks and surface raiders (battleships, heavy cruisers and disguised merchantmen), but the limited success of the surface fleet even after its choice of bases was expanded by the capture of Norway and France in 1940 fairly rapidly led to concentration on a U-boat based strategy.

The British response was to set up a system of convoys (as had been done during the latter years of World War I) escorted by units with anti-submarine capabilities, initially destroyers - particularly older classes and the fifty old flush-deckers supplied by the USA on Lend-Lease - and converted trawlers, but later on cheaper specialist units such as corvettes (the Flower class), frigates (the River class) and escort destroyers (the Hunt class). From the American entry into the war - or indeed slightly before - US Navy destroyers also played their part.

The balance ebbed and flowed with improvements in technology and tactical doctrine on both sides - for the allies improved radar and asdic for detection, anti-submarine weapons like the Hedgehog and Squid depth charge projectors, and the breaking of the Germans' ciphers by Ultra, while the Germans developed wolfpack tactics, the snorkel and finally closed cycle H2O2 engines, but by 1943 the Germans seemed to be gaining the upper hand, with unsustainable losses to the allies; however increased air cover as longer range planes and escort carriers became available, as well as the production capacities of American shipyards and further gains in the cryptographic war, finally swung the advantage towards the allies.

Casualties on both sides were severe. By the end of the war, some 85% of German U-boat crews had been killed or (more rarely) captured. 30 000 seamen from the British merchant navy (which employed crews from all parts of the world, especially the parts with low rates of pay) and 7000 on ships flying the American flag lost their lives; for the Americans it was a higher casualty ratio than that of any of the combatant forces except the US Marines. As non-combatants, the seamen were not granted the rights of those in the armed forces - in British service a seaman whose ship was sunk had no right to pay after the day of the sinking, and there was no paid leave. Their widows were denied the additional benefits paid to widows of serving members of the armed forces, and their names rarely appeared on war memorials (although there is a memorial to them on Tower Hill, London). The USA finally recognised its merchant seamen as war veterans in 1988; the UK has yet to do so.


Chronology to follow when time permits and my books are no longer all in boxes.

Sources (apart from, like, stuff I know):
http://home.onet.co.uk/~canonesa
http://www.lascars.co.uk/war.html
http://www.infoplease.com/ipa/A0872853.html
The term "Battle of the Atlantic" could be considered something of a misnomer. After all, it fits most peoples' idea of a military campaign instead: it was a years-long conflict, spanning millions of square kilometers. However, the technologies and intent of the entire conflict in the Atlantic Ocean made what would normally be a campaign become, for all practical purposes, a single battle of incredible length and breadth. For five long years the North Atlantic was a single, tremendous battlefield where ships and boats from Allies and Axis fought in one of the most hostile environments on earth.

Outbreak

During the First World War, Germany had of course made great use of unrestricted submarine warfare, wreaking havoc among Allied shipping. Submarine warfare had nearly knocked the United Kingdom, which was nowhere near self-sufficiency and extremely dependent on resupply by sea. In the event of a second world war, the Germans reasoned, the main strategy would be to enact a submarine blockade of the island and wait things out on the Continent for London to be starved into submission.

There was, however, the matter of the tremendous disparity between the Royal Navy and the Kriegsmarine on the surface. The German navy was far and away not up to the task of taking on the British in a fleet action like the Battle of Jutland from the previous war. Karl Dönitz, commander of the German U-boat service in the months leading up to the war, sought to find a way to redress this balance. The British outnumbered the Kriegsmarine 5:1 in battleships and battlecruisers, roughly eight to one in cruisers both heavy and light, and nine to one in destroyers; obviously, a surface action was out of the question. The answer was to take the upcoming sea battle below the surface by using a large submarine corps. Dönitz and others, expecting the war to begin sometime in 1942 instead of 1939, had hoped for a fleet of some 250 U-boats and a number of major surface combatants with which to create an impenetrable blockade. However, Adolf Hitler had his own way instead, and Germany went to war in September 1939 with a pitiful surface fleet and fewer than fifty submarines. As some needed to be kept at home for training, and some were going to be in dock for refueling and repairs, only seven to ten boats could be at sea at any given time.

First Engagements

The Germans did not initially intend on engaging in unrestricted submarine warfare, where any ship in the combat area, not just military ships of the belligerent countries, was subject to sinking without warning. However, they ended up thrusting it upon themselves when Oberleutnant Fritz-Julius Lemp of U-30 mistook the British ocean liner SS Athenia for a warship and sent her to the bottom. One of the first blows in the war was therefore the first blow of what would become the Battle of the Atlantic.

The first few months of war were a confused melee, with German and British ships, aircraft, boats, and minefields springing up or being sent just about everywhere. Almost immediately the English Channel was blocked off twice: a British minelaying effort forced U-boats to enter the North Atlantic by sailing around Scotland, while German minefields in the Channel and the Thames approaches cut southeastern England off from shipping, sinking almost three dozen ships in a couple of weeks. Unfortunately, the Channel quickly became moot: Norway and France both fell to the Germans in June 1940. The results were twofold: first, Germany had an uninterrupted line of potential ports and airbases running from its own bases to the Barents Sea, letting its ships enter the North Atlantic by passing over Scotland. In case that wasn't bad enough, the surrender of France gave Germany access to French ports on the Bay of Biscay. So, in addition to a massively increased submarine-building program, Hitler's navy now had access to several major Atlantic ports, many of which were out of range of many Allied aircraft. Losses to Allied shipping were significant by this point; they were about to get obscene.

The Happy Time

For awhile after the battlelines were set in Europe, the Germans and Allies stumbled into a position which was quite advantageous for the German fleets. The Allies were still shaking down the convoy system, and were often running single ships across the Atlantic without any escort whatsoever. When they did run, Allied convoys were strained because of the need for warships in other fronts, and convoys of forty or fifty ships would occaisionally run with little more than a single armed trawler or corvette as escort. (This escort policy led to the quick end of the Children's Overseas Reception Board program after the torpedoing of the Volendam and the sinking of the City of Benares, but that's another story.) To make things just that little bit worse, asdic systems on Allied ships could only detect submerged U-boats. The Germans solved this problem by attacking from the surface at night, where surprise could be maximized against blacked-out ships using useless sonar. On top of this, the introduction of wolfpack tactics meant that convoys of forty or fifty ships would routinely come under attack by not just one but dozens of U-boats.

This whole era became known as the Glückliche Zeit, or the Happy Time, to German submariners, as they racked up appalling scores. On average, during this time every German U-boat which put to sea sunk eight merchantmen, and many U-boat captains such as Otto Kretschmer and Günther Prien became "aces" celebrated through the Axis nations with dozens of sinkings. It wasn't until the beginning of 1941, with improved air cover near shore, the Lend-Lease Program between Britain and the United States, and increasing use of dedicated naval escorts such as the Flower-class corvettes began to make things dangerous enough for the U-boats that kills began to level off. However, it would only be a temporary respite. After a brief comeback through the spring and early summer of 1941, the German fleet regained the initiative in an unexpected manner.

More Trouble for the Allies

One would expect the entry of the Soviet Union and the United States - albeit against their will - into the war to have been a disaster for the German fleet. After all, two great industrial powerhouses would be pouring supplies into the conflict. Although that would eventually be the case, the entry of the two great powers into the war reminded the U-boat captains just what the term "target-rich environment" really meant, plunging them headlong into a second Happy Time.

In the east, Operation Barbarossa led not only to German forces rolling over the Soviets; it led to a desperate need by the Soviets for supplies of every kind to try and halt the invasion. Overland routes through Europe were of course foolish, and the land route through Iran required getting shipping there in the first place, which required navigating either the Mediterranean Sea, which was crawling with U-boats and Italy's naval forces, or taking the long and treacherous route around the Cape of Good Hope, which of course carried its own risk. The only other option, short of leaving the Soviets to their own devices, was the most direct sea route possible, from Great Britain or Iceland to the Soviet ports of Murmansk and Arkhangelsk. This route, which was known as the Murmansk Run, was probably the single most dangerous route seamen or pilots could possibly take part in. The fact that the weather in the Norwegian Sea and Barents Sea was legendarily bad notwithstanding, making the Murmansk run involved spending day after day within easy reach of German shore-based aircraft, surface raiders from pocket battleships on down, and of course the ubiquitous U-boat. Allied forces had little to protect themselves aside from luck, what weapons ships could carry, and the occaisional aircraft from CAM ships, which catapaulted their single fighter into the air to defend against bombers or submarines and would then ditch in the ocean, possibly to await pickup. Life expectancy in the frigid waters of the Barents Sea was less than two minutes.

The Murmansk run saw several of the largest convoy battles of the war, where large flotillas of Allied freighters and escorts would engage in combat with multiple converging wolfpacks in addition to surface and air threats. For a year and a half the German forces had by far the upper hand. Convoys would often arrive with less than a third of their original tonnage remaining. One convoy, "PQ-17" from Iceland to Murmansk, lost twenty-four ships out of roughly forty attached to it. With those sunken ships went 210 aircraft, 430 tanks - enough to equip three to five armoured divisions to full strength - 3,350 other vehicles, one hundred thousand tons of general cargo, and 153 sailors. In their haste to make it back to the relative safety of Britain, the returning "QP" convoys would often blunder into pack ice, additional raids, or their own forces' minefields to take even further losses. The runs past Norway kept the Soviet Union supplied, but at grievous cost in ships, men, and equipment which the Allies could not maintain.

Halfway around the world, the United States was recieving a rude awakening to the coming of the war on their Atlantic coast, even as they began preparing for the relentless drive through the Pacific Ocean. American ships had been trading fire halfheartedly with German submarines since May 1941, and wholeheartedly since the end of October, when the USS Reuben James was sunk by U-562. By midyear the British and Americans had divided the Atlantic into strategic zones of east and west, with the British having control over everything east of the Azores while the United States directed its own naval operations, as well as nominally managing those of Canada, which by 1942 was engaging in the lion's share of escort duties. However, the United States Navy was hesitant to change operating procedures when the war broke out, and actively refused to form convoys with escorts, or black out port cities at night, for several months after their declaration of war on Germany. German submarines, which flocked to the American coastline to find targets with the assistance of "milch cow" tanker submarines, found large, unescorted ships, lit up and silhouetted by large cities.

American and Allied shipping in the Gulf of Mexico, Caribbean Sea and the eastern seaboard were devestated, and further north German submarines pressed the Canadian navy and merchant marine harder by initiating the Battle of the Gulf of St. Lawrence. In 1942, the appalling count of 1,664 merchant ships were sunk in the North Atlantic alone, 1,160 of those at the hands of submarines, for a total of 7,790,000 tonnes of shipping and uncountable mountains of supplies. At the height of this period, U-boats were operating in the Atlantic and the Norwegian Sea in packs of up to forty boats equipped with longer ranges and better weapons and able to attack with relative impunity.

The Tide Turns

In the end, as is often the rule in modern war, it was the man with the screwdriver or welding mask, and not the man with the lanyard or bomber controls, which settled the outcome of the Battle of the Atlantic. By mid-1943 Allied military production was finally beginning to overtake that of Germany. With the Germans being driven back on the Eastern Front, the Soviet Union had breathing room, and it was the Germans' turn to worry about supply. The famous American Liberty ship program was in full swing, kicking out seemingly infinite numbers of the freighters in unbelievably short amounts of time (the record was five days!). At the worst points of the battle, one ship of 10,000 tons displacement was being sunk every ten hours for period of just over a month; by late 1943, however, the Allies were building ships faster than the Germans could sink them. They were also sinking U-boats faster than the Germans could build them.

The Battle of the Atlantic was as much a war of technology as anything else; fortunes in the conflict shifted back and forth according to who was ahead in reseach, intelligence, and cryptography. Before 1943, the Germans were winning, often devestatingly so, with several convoy routes being practical death sentences. By the latter half of the war, however, the Allies were gaining the edge in every way. Convoys in the late war were lavishly protected relative to the waddling target conventions which made up the first years. Beginning in mid to late 1942, escort carriers began to ply the major sea lanes, providing air cover in the "black pits" of the central Atlantic, previously out of range of any air cover. Ships were now equipped with better sonar, as well as radar accurate enough to see a periscope at a range of nine kilometers or more. Escorting ships carried more and more deadly weapons, such as the "Hedgehog" and "Squid" ASW mortar systems and improved depth charges, and were covered by flocks of ASW aircraft making use of new weapons such as the "Mk. 24 Mine," which was actually the first homing torpedo deployed in war. Codebreaking featured more and more prominently as well, as Allied intelligence officers took advantage of captured Enigma machines and years of gathered information to repeatedly break Germany's encrypted communications. The Germans repeatedly broke Allied channels as well, but by the time that became routine, the Allies could afford to absorb losses.

At last, the wolf packs could be directly challenged and beaten by Allied navies. In April-May 1943, a week-long running battle between thirty U-boats and the heavily-escorted convoy ONS-5 resulted in nineteen submarines sunk or crippled. Although the U-boats sunk twelve merchant ships, it would be their last costly action. Dönitz, horrified at mounting losses, called an end to the U-boat campaign, pulling most of his boats out of the North Atlantic. A little over a year later, the liberation of France after Operation Overlord lost Germany its Atlantic ports. Aside from scattered, ineffectual raiding, the Battle of the Atlantic was over by August 1944, although the ONS-5 battle's end on May 5, 1943 is considered to be a better candidate for the campaign's finish.

Conclusion

The U-boat campaign was one of the largest naval battles in history, possibly second only to the huge actions of the Pacific Theatre. All told, German attacks were responsible for the loss of roughly five thousand Allied ships, including some two hundred warships, of some 22,000,000 tons' total displacement between September 3, 1939 and May 5, 1943. These ships took somewhere between thirty and forty thousand crewmen to the bottom with them. However, losses were light compared to the Germans'. Of the roughly 1,160 submarines built by Germany over the course of the war, approximately eight hundred were sunk, scuttled, or damaged to the point of total loss. 40,600 personnel served aboard the U-boats; perhaps one in five lived to see the end of the war. We tend to see history through hindsight which gives an air of inevitability to things: "Of course the Germans were doomed!" However, it is not always so clear-cut. Almost until the final major battles, the U-boat arm inflicted tremendous losses on Allied fleets, and came very close to cutting off supplies to Britain altogether in the early years. Winston Churchill would later admit that, of all the weapons Germany used in the war, the U-boat was the only one he truly feared. The cordon never fully closed, however, and one hundred eighty million tons of supplies got through the submarine line to Great Britain alone throughout the war.

Legacies of the Battle of the Atlantic continue to the present day, mainly in terms of defining just who "fought" in it and who was simply unlucky enough to be shot at in it. Merchant mariners from all the Allied countries have fought, often for decades, to have their service recognized as just that - service - to recieve the recognition of their status as veterans. It is difficult to see how they could be viewed as anything but military service; the convoys' crews served in what one could consider perpetual combat conditions, waiting for the explosion of a torpedo striking their ship or their neighbour's, defending their ships using what meagre weapons were at their disposal, and, if the worst happened, wondering whether it would be simpler to just dive into the Atlantic and end it all rather than linger for days or weeks on lifeboats. The crews of those waddling, often defenseless freighters did at least as much to win the war as their escorts and the men at the front they helped supply. It is one of history's tragedies that they have only recently begun to be formally honoured for their actions and sacrifices.

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