37.1 KT4 - War at sea: submarines, air power and codes, 1917-45

Enquiry Frame

This is a Paper 3 depth key topic. The specification is not asking for an isolated story of one famous battle. It asks you to compare how sea war worked across two world wars and across two oceans.

Use this enquiry structure:

The chronological spine is short but demanding:

The phrase complex codes needs careful handling. In ordinary speech, people often say "the Enigma code". More precisely, Enigma was a cipher machine used to encipher messages. For exam purposes, the key point is not the technical difference between a code and a cipher. The key point is that modern sea war depended on radio communication, and radio communication could be intercepted. The side that could exploit enemy signals while protecting its own gained a major operational advantage.

Keep one warning in mind throughout the lesson: do not say "technology won the war at sea" as if machines acted by themselves. The strongest answers explain combinations. Convoys without escorts were vulnerable. Radar without aircraft had limited reach. Decrypts without ships could not sink a U-boat. Carriers without good reconnaissance could be surprised. Sea war between 1917 and 1945 was a contest between whole systems.

Submarine Crisis, 1917-18

Germany's U-boat campaign in 1917 was a gamble to knock Britain out of the First World War before the United States could intervene effectively. Britain was an island state dependent on food, fuel, raw materials and munitions moved by sea. The Royal Navy's blockade also hurt Germany badly. If Germany could sink merchant shipping faster than Britain and its allies could replace it, Britain might be forced to seek peace.

Earlier in the war, U-boat warfare had been restricted by "prize rules". A submarine was expected to surface, stop a merchant vessel, inspect it, and allow crew and passengers to escape before sinking it. That was dangerous for submarines. A surfaced U-boat was fragile; it could be rammed, shelled or trapped by disguised armed merchant ships known as Q-ships. Germany had already used unrestricted submarine warfare in 1915, but the sinking of passenger liners such as Lusitania helped turn neutral opinion, especially American opinion, against Germany.

The 1917 decision was therefore a calculated risk. On 1 February 1917, Germany resumed unrestricted submarine warfare. The German leadership knew this might bring the United States into the war. Its assumption was that Britain could be starved out before American military power arrived in Europe in decisive strength.

The initial impact was alarming. U-boats attacked merchant ships in the Atlantic and around the British Isles. In the spring of 1917, shipping losses rose sharply. Britain was not only losing cargoes; it was losing ships, trained merchant crews and confidence in the security of the supply system. Submarine warfare also had a psychological impact because it made the sea itself feel unsafe. A hidden attacker could destroy a ship without warning, far from any battlefield.

The British response was slow because the Admiralty had objections to convoy at first. Some officers feared that grouping merchant ships would create a larger target, delay sailings, overload ports, and require more escorts than Britain possessed. There was also an older naval habit of thinking in terms of hunting submarines independently rather than organising merchant shipping around protection.

Convoy worked because it changed the mathematics of submarine warfare.

Convoy was not the only counter-measure. Britain also used depth charges, hydrophones, dazzle camouflage, Q-ships, patrol craft, mines, aircraft patrols and direction-finding. Some measures were more effective than others. Q-ships had dramatic successes but could not protect shipping at scale. Dazzle camouflage did not make ships invisible, but it could make it harder to estimate course and speed. Depth charges mattered because escorts needed a weapon that could attack a submerged submarine. Aircraft were still limited in range and payload, but they forced U-boats to be more cautious near coasts and convoy routes.

The result was eventual British success. The U-boat campaign did not fail because submarines were weak. It failed because Germany's window of opportunity closed. The United States entered the war on 6 April 1917, convoy reduced losses, shipbuilding and imports continued, and Britain avoided starvation. By 1918, German submarines were still dangerous, but they no longer looked capable of deciding the war before Germany's wider strategic position collapsed.

For judgement, the key contrast is between initial tactical triumph and strategic failure. In early 1917, U-boats were tactically effective because they sank large amounts of shipping. Strategically, the campaign backfired because it helped bring in the United States and forced Britain to adopt the convoy system that made submarine warfare much less decisive.

Dönitz and the Atlantic War

The Second World War reopened the submarine problem. Germany again understood that Britain could be weakened by cutting its Atlantic lifeline. Food, oil, munitions, raw materials, troops and later American Lend-Lease supplies all depended on ocean routes. If Germany could sever the connection between North America and Britain, Britain might be isolated and any future Allied invasion of western Europe would become impossible.

Karl Dönitz was central to this strategy. He had served as a U-boat officer in the First World War and became commander of the German U-boat arm before becoming commander-in-chief of the German navy in 1943. Dönitz believed that submarines offered Germany the best chance of damaging Britain because Germany could not match the Royal Navy's surface fleet. His aim was a tonnage war: sink merchant ships faster than the Allies could replace them.

The main tactical development was the wolf pack. A single U-boat could find a convoy but might not be able to destroy it alone. Instead, a submarine that located a convoy could report its position by radio to headquarters, and other U-boats could be directed to gather and attack, often at night and on the surface. This used the submarine less as a lone assassin and more as part of a coordinated striking force.

The Battle of the Atlantic went through phases:

Dönitz's strategy had strengths. U-boats were cheaper than battleships, hard to detect, and able to threaten civilian and military supply at the same time. Radio coordination meant that a convoy sighting could be exploited by many boats. Night surface attacks reduced the advantage of early sonar, because ASDIC was designed to detect submerged submarines and was less useful against a U-boat running fast on the surface.

But the strategy also had weaknesses. U-boats depended on radio messages, and those messages could be intercepted. Submarines had to surface regularly to recharge batteries and travel efficiently. A convoy protected by escorts, aircraft and intelligence was a far harder target than a lone merchant ship. Germany also faced an industrial problem: if Allied shipyards built more merchant ships than U-boats could sink, and if Germany lost trained submariners faster than it could replace them, the tonnage war would fail.

The Battle of the Atlantic should therefore be judged as a battle of attrition and adaptation. Dönitz did not need to sink every ship. He needed to sink enough ships for long enough to break Britain's war economy. The Allies did not need to eliminate every U-boat. They needed to keep the sea lanes open, reduce losses, and make U-boat operations too costly. That is why May 1943 mattered so much: it showed that the balance of adaptation had shifted.

May 1943, Codes and Counter-Measures

May 1943 is the official turning point because Dönitz's U-boats lost the initiative in the North Atlantic. The crisis had been very real. In March 1943, convoy battles such as HX229/SC122 inflicted heavy merchant losses and made Allied leaders fear that the Atlantic lifeline was still in danger. Yet within weeks, the position changed. Convoy ONS 5, attacked from late April into early May, lost merchant ships, but U-boat losses were also severe. By 24 May 1943, Dönitz ordered most U-boats away from the North Atlantic convoy routes because losses had become unsustainable.

May 1943 was not a magic date when submarines stopped mattering. U-boats returned, new weapons appeared, and the Battle of the Atlantic continued until 1945. The significance of May 1943 is that Germany never again came as close to breaking the Atlantic supply system. From this point, U-boats were increasingly forced to operate under Allied air, surface and intelligence pressure.

The Allied success came from a combined counter-submarine system: