Unrestricted submarine warfare

1. Lead

A weapon that changed the rules of war

Unrestricted submarine warfare is the practice of attacking merchant shipping without warning and without first ensuring the safety of the crew and passengers, as the traditional rules of war at sea required. A submarine that practised it did not stop a merchant ship, search it, and allow its crew to take to the lifeboats; it torpedoed the ship on sight, often at night and often without surfacing, and left the survivors to the sea.

The tactic was first used on a large scale by Germany in the First World War, and it was used again, far more systematically, in the Second World War, in the long campaign that became known as the Battle of the Atlantic.

In both wars, it was one of the most controversial tactics of the conflict, because it killed civilians — including the passengers of liners such as the Lusitania — and because it challenged the distinction between combatants and non-combatants on which the laws of war depended.

Why it mattered

Unrestricted submarine warfare mattered for two reasons, one military and one political. Militarily, it was the weapon of a weaker naval power against a stronger one: Germany could not challenge the British Royal Navy on the surface, but its U-boats could strangle Britain’s supply lines and starve the island into submission.

Politically, it was a weapon that repeatedly backfired, because the sinking of neutral and passenger ships outraged public opinion in the United States and, in the First World War, was one of the main reasons the United States entered the war in 1917.

In the Second World War, the campaign was longer, more destructive and more evenly matched, and it was decided, in the end, not by the U-boats but by the Allied convoy system, by radar and sonar, and by the sheer industrial output of American shipyards.

The story of unrestricted submarine warfare is the story of a tactic that came close to winning two wars — and that, in the end, lost both of them.

2. Key Facts

Definition	Sinking merchant ships without warning and without ensuring the safety of crews and passengers
First large-scale use	Germany, First World War, February 1915 (declared war zone around Britain)
Most famous incident	Sinking of the liner Lusitania, 7 May 1915, about 1,200 dead
Second World War	German U-boat campaign in the Atlantic, 1939–1945; United States declared unrestricted submarine warfare against Japan on 7 December 1941
Allied answer	The convoy system, escort carriers, radar, sonar (ASDIC), long-range aircraft, and code-breaking
Cost / Cái giá	About 3,500 Allied merchant ships sunk in the Second World War (about 14–15 million tons); about 30,000 Allied merchant seamen killed; about 780 German U-boats lost of about 1,150 built

3. Background

The law of the sea before the submarine

Before the First World War, the rules for attacking merchant ships at sea were set out in the prize rules, a body of law that had developed over centuries. A warship that wished to sink an enemy merchant ship was expected to stop it, search it, and, if it decided to sink it, to take the crew to safety first — in the ship’s own lifeboats, or on board the warship.

The rules were designed for a world in which warships fought on the surface, in which a merchant ship could be stopped and boarded, and in which the distinction between a warship and a merchant ship was clear. The submarine broke all three assumptions.

A submarine was small, fragile and vulnerable on the surface, and if it surfaced to stop a merchant ship it risked being rammed or shelled by the merchant ship’s guns. A submarine could not take on board the crew of a ship it had sunk. And a merchant ship, once armed, could itself become a warship in all but name.

The submarine, in short, could not obey the prize rules and survive, and the question of what should replace them was never settled before the First World War began.

Germany’s naval dilemma

Germany entered the First World War with the second-largest navy in the world, but it was still far smaller than the British Royal Navy, and it could not win a surface battle for command of the sea. The German High Seas Fleet, after the Battle of Jutland in 1916, remained in port for most of the war, while the British blockade slowly strangled the German economy.

The submarine offered a way out. Germany had built a fleet of U-boats, and its naval commanders argued that the U-boat could do what the surface fleet could not: it could attack British trade directly, sink the ships that carried food and raw materials to Britain, and force the island to make peace. The argument was compelling, but it carried a risk that the German leadership understood from the start.

Britain’s food supply depended on the sea, but so did the goodwill of the United States, and the sinking of American ships, or of ships carrying American passengers, might bring the United States into the war. The history of unrestricted submarine warfare in the First World War is the history of Germany weighing that risk, deciding, twice, that it was worth taking — and being proved wrong the second time.

4. Opposing Forces

The U-boat and its hunters

The weapon at the centre of unrestricted submarine warfare was the U-boat, and its character changed greatly between the two world wars. The U-boats of the First World War were small, slow and short-ranged, and they could carry only a handful of torpedoes.

The U-boats of the Second World War — above all the Type VII, the workhorse of the German fleet, of which more than 700 were built — were larger, faster on the surface, and able to stay at sea for weeks.

But the U-boat was, in both wars, a surface vessel that could submerge, rather than a true submarine: it travelled on the surface, using its diesel engines, and it submerged only to attack or to escape, running on electric batteries that lasted a few hours. It was most dangerous at night, on the surface, when it could see a convoy’s ships silhouetted against the sky and attack from among them.

Its great weakness was that it had to surface to recharge its batteries and to travel at speed, and it was on the surface, or just below it, that it was most vulnerable to aircraft and to escort ships.

The two sides of the Atlantic campaign

	Germany (U-boat arm)	Allies (convoy defenders)
Commander	Grand Admiral Karl Dönitz (from 1939, as Flag Officer U-boats; from 1943, Commander-in-Chief of the Navy)	Admiral Sir Max Horton (Western Approaches Command, from 1942); Admiral Chester W. Nimitz (US Pacific Fleet)
Main weapon	Type VII and Type IX U-boats; the “wolfpack” tactic	Convoys, escort destroyers and corvettes, escort carriers, long-range aircraft
Numbers	About 1,150 U-boats built during the war; about 780 lost	Thousands of merchant ships; hundreds of escorts; by 1943, air cover across most of the Atlantic
Key technology	Torpedoes, including acoustic homing torpedoes late in the war; snorkel (from 1944)	Radar, sonar (ASDIC), high-frequency direction finding (HF/DF, “Huff-Duff”), Ultra code-breaking, depth charges and the Hedgehog

The balance between the two sides shifted decisively in 1943. In the first years of the war, the U-boats had the advantage: the Allies had too few escorts, too few aircraft, and no air cover in the middle of the Atlantic, the so-called “Black Pit”. In 1943, the Allies closed the gap — more escorts, escort carriers, very-long-range aircraft, and, crucially, the ability to read German naval signals through Ultra — and the U-boat losses became unsustainable. Dönitz withdrew his boats from the North Atlantic in May 1943, in what the Germans called “Black May”, and the campaign never recovered.

5. The Battle

The First World War: the Lusitania and the decision of 1917

Germany declared a war zone around Britain in February 1915, and U-boats began sinking merchant ships without warning. The most famous victim was the British liner Lusitania, torpedoed off the coast of Ireland on 7 May 1915 with the loss of about 1,200 lives, including 128 Americans.

The sinking caused outrage in the United States, and Germany, fearing American entry into the war, suspended unrestricted submarine warfare and returned to the prize rules. But the pressure from the German navy, and the belief that the U-boat was Germany’s only hope of winning the war, did not go away.

In January 1917, the German leadership decided to resume unrestricted submarine warfare, in the belief that Britain could be starved into submission within six months, before the United States could raise and transport an army to Europe. The calculation was wrong on both counts.

Britain did not starve — the convoy system, introduced in the spring of 1917, cut shipping losses sharply — and the United States declared war on Germany on 6 April 1917, citing unrestricted submarine warfare as one of its principal reasons. The German gamble had brought into the war the one power whose industrial strength would, in the end, decide it.

The convoy answer

The convoy system, introduced by Britain in the spring of 1917, was the answer to the U-boat that the prize rules had never provided. Merchant ships sailed together, in groups of dozens, escorted by warships, and the U-boat, instead of picking off single ships at leisure, had to attack a defended group. Losses fell sharply, and by the end of the war the U-boat campaign had failed. The lesson of 1917 — that the convoy, not the single ship, was the unit of survival at sea — would be relearned, at great cost, in the Second World War.

The Second World War: the wolfpacks

The Second World War U-boat campaign began on the first day of the war, when a U-boat sank the liner Athenia on 3 September 1939, and it continued, with varying intensity, until the last days of the war in May 1945.

Its most dangerous phase was the period from 1940 to 1943, when Admiral Karl Dönitz’s “wolfpack” tactic — groups of U-boats, directed by radio from headquarters in France, converging on a convoy at night and attacking from the surface — sank merchant ships faster than the Allies could build them.

The worst months were the “Happy Time” of 1940–41, when U-boats operated off the American coast with almost no opposition, and the winter of 1942–43, when the losses in the North Atlantic reached their peak. In March 1943, the worst month of the campaign, the U-boats sank more than 100 Allied ships. But the Allies were learning.

The convoy system, the escort carriers, the long-range aircraft that closed the “Black Pit”, and the code-breakers at Bletchley Park who read the German naval Enigma — all of these came together in the spring of 1943, and in May 1943 the U-boats lost 41 boats in a single month, a rate of loss that Dönitz could not sustain.

He withdrew from the North Atlantic, and although U-boats continued to operate until the end of the war, the campaign had been decided.

The Pacific war

Unrestricted submarine warfare was not only a German tactic. On 7 December 1941, the day of the attack on Pearl Harbor, the United States Navy ordered its submarines to conduct unrestricted submarine warfare against Japan. The American campaign, fought by a force that grew from a handful of boats to more than 200 submarines, was devastatingly effective: American submarines sank about 55 per cent of all Japanese merchant shipping lost in the war, and they strangled the Japanese economy, which depended on imports of oil, food and raw materials.

The campaign was also, by the standards of the time, more ruthless than the German campaign in the Atlantic, because the Japanese convoy system was weaker, Japanese anti-submarine warfare was less effective, and American submarines were ordered, in the later stages of the war, to attack any Japanese ship, including small coastal vessels.

The Pacific campaign is a reminder that unrestricted submarine warfare was not a uniquely German crime, but a tactic that every major naval power adopted when it suited its interests — and that its victims, in every case, were overwhelmingly civilian merchant seamen.

6. Turning Point

May 1943: Black May

The turning point of the U-boat war came in May 1943, in a month the Germans called “Black May”. In that month, the U-boats lost 41 boats — about a quarter of the boats at sea — while sinking far fewer ships than in the months before.

The losses were the result of several Allied advances coming together at once: the escort carriers, which provided air cover over the convoys; the very-long-range Liberator aircraft, which closed the mid-Atlantic “Black Pit”; the centimetric radar, which could detect a surfaced U-boat at night; the high-frequency direction finders, which located U-boats by their radio transmissions; and the code-breakers, who read the German naval signals and routed the convoys around the waiting wolfpacks.

On 24 May 1943, Dönitz withdrew his U-boats from the North Atlantic. The withdrawal was temporary — the boats returned, in smaller numbers and with new equipment, later in the war — but the initiative had passed to the Allies, and it never returned. The U-boat war had been won, not in a single battle, but in a month of attrition in which the hunters became the hunted.

The technology that decided it

The deeper turning point was technological, and it was decided in the laboratories and the shipyards rather than at sea. The Allies’ victory in the Atlantic was, in the end, a victory of industrial production and of science: the United States built merchant ships — the Liberty ships — faster than the U-boats could sink them, and it built escorts, aircraft and radar sets in quantities that Germany could not match.

The German response — the snorkel, which allowed a U-boat to run its diesels while submerged, and the Type XXI, a true submarine that could travel submerged at high speed — came too late, and in too few numbers, to change the outcome.

The Type XXI, of which more than 100 were built but only a handful saw service, was a revolutionary design, and it became the model for post-war submarines around the world. But it arrived in 1945, when the war was already lost, and it is a reminder that in a long war, the side that can produce more, and learn faster, usually wins.

7. Casualties and Losses

The merchant seamen

The human cost of unrestricted submarine warfare fell above all on merchant seamen, the civilian sailors who carried the supplies on which the war depended. In the Second World War, about 3,500 Allied merchant ships were sunk by U-boats, and about 30,000 Allied merchant seamen were killed — a death rate, as a proportion of those who served, that was higher than that of any of the armed services except the U-boat arm itself.

The figures are approximate: the records of merchant shipping are incomplete, and different sources give different totals, but the order of magnitude is not in dispute. The merchant seamen died in the cold water of the North Atlantic, often far from help, and often after surviving one sinking only to be sunk again.

Their story was, for many years, little known, because they were civilians and because their deaths were recorded ship by ship rather than battle by battle. In Britain, the Merchant Navy’s losses were commemorated at Tower Hill in London, and in the United States the merchant mariners who died were not given veteran status until 1988.

The U-boat crews

The U-boat crews paid the highest price of any service in the war. Of the about 40,000 men who served in German U-boats, about 28,000 were killed — a death rate of about 70 per cent — and of the about 1,150 U-boats built, about 780 were lost.

The U-boat arm was, by the end of the war, a service in which a young officer could expect to die on his first or second patrol, and in which the boats that returned to port were the exception rather than the rule. The figures are well documented, from German naval records, and they are not in serious dispute.

The contrast between the two sides is stark: the merchant seamen died because they were the targets of a tactic that made no distinction between combatants and civilians, and the U-boat crews died because the tactic, once the Allies had learned to counter it, became a form of suicide. Both groups deserve to be remembered, and neither should be forgotten in the telling of the campaign.

8. Map and Geography

The Atlantic battlefield

The battlefield of unrestricted submarine warfare was the North Atlantic, the stretch of ocean between North America and Britain, across which the supplies that kept Britain in the war had to travel. The convoys sailed from Halifax in Canada and from New York, and they followed the great circle route to Britain, passing south of Greenland and Iceland, and then through the Western Approaches to Liverpool, Glasgow and the Clyde.

The U-boats waited for them in the middle of the ocean, in the “Black Pit” — the area beyond the range of land-based aircraft from either side, where the convoys were without air cover. The bases from which the U-boats operated were, after the fall of France in 1940, on the French Atlantic coast — at Brest, Lorient, Saint-Nazaire, La Rochelle and Bordeaux — and the concrete pens in which they sheltered, some of which still stand today, were among the largest structures built by the Germans during the war.

The geography of the campaign was the geography of distance: the Atlantic was wide enough for the U-boats to hide in, and narrow enough, at the Western Approaches, for the convoys to be found.

The convoy routes and the Black Pit

The “Black Pit” was the key to the campaign, and its closing was the key to the Allied victory. The gap in air cover, in the middle of the Atlantic, was about 600 kilometres wide, and it was in that gap that the wolfpacks did their worst work.

The Allies closed it in two ways: by building very-long-range aircraft, the Liberators, that could fly from Newfoundland and from Northern Ireland and meet in the middle, and by building escort carriers, small aircraft carriers that sailed with the convoys and provided air cover all the way across.

By the summer of 1943, the gap was closed, and the U-boats, which had once hunted in safety in the middle of the ocean, were hunted in turn. The map of the Atlantic campaign is, in the end, a map of air cover: where the aircraft could fly, the convoys survived; where they could not, the U-boats ruled.

9. Outcome and Legacy of Unrestricted Submarine Warfare

The campaign that failed — twice

Unrestricted submarine warfare failed in both world wars, and it failed for the same reasons. In the First World War, it failed because the convoy system defeated it and because it brought the United States into the war. In the Second World War, it failed because the convoy system, the aircraft and the code-breakers defeated it, and because the United States, once in the war, built ships faster than the U-boats could sink them.

The tactic came close to success in both wars — in 1917, Britain’s food stocks fell to a few weeks’ supply, and in early 1943 the Allied shipping losses were, for a time, greater than the rate at which new ships were being built — but in both cases the margin was narrow, and in both cases the Allies found the answer in time.

The lesson, for naval strategists, was that the submarine, for all its power, could not win a war against a power that controlled the sea from the air and from the surface, and that could out-produce its enemy.

The law of the sea after the wars

The legacy of unrestricted submarine warfare is also a legal one. After the First World War, the attempt to ban the submarine, or to restrict it to the prize rules, failed: the Washington Naval Treaty of 1922 and the London Naval Treaty of 1930 both tried to impose the prize rules on submarines, and both were ignored when war came again.

After the Second World War, the question was settled differently: at the Nuremberg trials, Admiral Dönitz was charged with waging unrestricted submarine warfare, but he was not convicted on that charge, in part because the prosecution could not show that the Allies themselves had not done the same — the American campaign against Japan was, by any definition, unrestricted submarine warfare.

The modern law of the sea, as set out in the Geneva Conventions and their Additional Protocols, prohibits attacks on civilian ships except in narrowly defined circumstances, but the practice of unrestricted submarine warfare, and the debate about it, has never entirely gone away.

It remains, in the history of war at sea, the tactic that most clearly exposed the gap between the laws of war and the realities of war.

10. Commanders

The German commanders

Grand Admiral Karl Dönitz (Germany). The commander of the German U-boat arm from 1939, and from January 1943 the Commander-in-Chief of the German Navy. Dönitz was the architect of the wolfpack tactic and the driving force behind the U-boat campaign. He was a U-boat commander himself in the First World War, and he understood the weapon better than any other senior officer. After Hitler’s death in April 1945, Dönitz became, briefly, the head of the German government, and he signed the surrender. At Nuremberg he was sentenced to ten years’ imprisonment. His memoirs, published after the war, remain one of the fullest accounts of the U-boat war from the German side.

Kapitän zur See Günther Prien (Germany). The commander of U-47, who sank the British battleship Royal Oak at anchor in Scapa Flow in October 1939, in one of the most daring submarine attacks of the war. Prien became a national hero in Germany, and his exploit was used in Nazi propaganda. He was killed in March 1941, when U-47 was lost with all hands. His death was not announced in Germany for two months.

The Allied commanders

Admiral Sir Max Horton (Britain). Commander-in-Chief of Western Approaches Command from November 1942, and the man who, more than any other, organised the Allied victory in the Atlantic. Horton, himself a submariner in the First World War, understood the U-boat, and he introduced the support groups — groups of escorts that were free to hunt U-boats rather than to stay with the convoys — and the tactics that, in the spring of 1943, broke the wolfpacks.

Admiral Chester W. Nimitz (United States). Commander-in-Chief of the United States Pacific Fleet, under whose command the American submarine campaign against Japan was fought. Nimitz, who had served in submarines early in his career, ordered unrestricted submarine warfare against Japan on 7 December 1941, and he defended that decision at Nuremberg, where his testimony helped to secure Dönitz’s acquittal on the unrestricted submarine warfare charge.

Captain Frederic “Johnny” Walker (Britain). The commander of the 2nd Support Group, and the most successful anti-submarine commander of the war. Walker’s group sank more U-boats than any other, and his tactics — the “creeping attack”, in which two escorts worked together to pin a U-boat down — became standard practice. He died of a stroke in July 1944, exhausted by the strain of the campaign.

Frequently Asked Questions

What is unrestricted submarine warfare?
The practice of sinking merchant ships without warning and without first ensuring the safety of their crews and passengers, in breach of the traditional prize rules of war at sea.

Did unrestricted submarine warfare bring the United States into the First World War?
It was one of the main reasons. Germany resumed unrestricted submarine warfare in January 1917, and the United States declared war on Germany on 6 April 1917, citing the U-boat campaign as a principal cause.

What was a wolfpack?
A group of German U-boats, directed by radio from headquarters ashore, that converged on an Allied convoy at night and attacked together from the surface. The tactic was developed by Admiral Karl Dönitz.

When was the turning point of the U-boat war?
May 1943, “Black May”, when the U-boats lost 41 boats in a single month and Admiral Dönitz withdrew them from the North Atlantic.

Did the United States also practise unrestricted submarine warfare?
Yes. On 7 December 1941, the United States ordered its submarines to conduct unrestricted submarine warfare against Japan, and the American campaign sank about 55 per cent of all Japanese merchant shipping lost in the war.

How many U-boats were lost?
About 780 of the about 1,150 U-boats built during the Second World War were lost, and about 28,000 of the about 40,000 men who served in them were killed.

Related articles

• Counterattack
• Flanking manoeuvre
• River crossing

Sources

1. Wikipedia, “Unrestricted submarine warfare” — the prize rules, the Lusitania, the 1917 decision, and the U-boat war.
2. Wikipedia, “Battle of the Atlantic” — shipping losses, the wolfpacks, and Black May 1943.
3. Karl Dönitz, Memoirs: Ten Years and Twenty Days (Weidenfeld & Nicolson, 1959) — the German commander’s account of the U-boat war.
4. Clay Blair, Hitler’s U-Boat War (Random House, 1996–1998) — the fullest modern history of the campaign, in two volumes.
5. Marc Milner, Battle of the Atlantic (Tempus, 2005) — the Allied convoy system and the victory of 1943.
6. US Naval History and Heritage Command — the American submarine campaign against Japan, 1941–1945.

Further reading

• Michael Gannon, Operation Drumbeat (Harper & Row, 1990) — the U-boats off the American coast in 1942.
• Terence Robertson, Walker, R.N. (Evans Brothers, 1956) — the life of Captain Frederic Walker.