Submarine warfare

• 1. Introduction

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  Submarine warfare began, by one account, on the 8th of September 1776. That day, the American submersible Turtle attempted an underwater attack on the British ship of the line HMS Eagle. The attack failed, but it stands as the first submarine attack ever recorded. Submarine warfare is one of four divisions of underwater warfare, alongside anti-submarine warfare, mine warfare, and mine countermeasures. It centers on diesel and nuclear submarines carrying torpedoes, missiles, or nuclear weapons. Advanced sensing equipment lets these vessels find and strike other submarines, ships, or targets on land. Submarines have also carried out reconnaissance, landed special forces, and served as tools of deterrence. Some navies use them to screen task forces as well. How effective any of this proves to be depends partly on the anti-submarine warfare mounted in response. How did that failed attempt against HMS Eagle grow into a weapon capable of reshaping entire wars? And what does submarine warfare look like today, generations after Turtle's failed dive?

• 2. Civil War Firsts

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  The 1860s brought a wave of new warship designs to both the United States and the Confederate States Navies. Submarines counted among the vessels invented for that fight, and this period marks the start of the age of submarine warfare. On the 17th of February 1864, one of those submarines slipped into Charleston Harbor, South Carolina, and sank a Union sloop. It was the first time in recorded history that a submarine had sunk an enemy ship.

  The submarine that struck the Union sloop in Charleston Harbor was a Confederate privateer. Shortly afterward, H. L. Hunley sank, taking her entire crew of eight down with her. Her one successful strike proved to be her last mission. The next major chapter in submarine warfare would not unfold in American coastal waters. It would come across the Atlantic, in a war fought at a scale far beyond Charleston Harbor.

• 3. The U-boat War

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  In February 1915, German U-boats launched their first major wave of attacks on Allied merchant ships headed for Britain, France, and Russia. American civilian deaths, most notably in the sinking of the Lusitania, turned public opinion in the United States against the Central Powers. Washington demanded Germany stop, and from September 1915 to January 1917 German submarines operated under prize rules, which required warning ships before attacking. Admiral Henning von Holtzendorff served as chief of the German admiralty staff and lived from 1853 to 1919. In December 1916, he argued successfully for a return to unrestricted attacks, meant to starve Britain into surrender. German commanders knew that resuming unrestricted attacks, now extended to neutral shipping, meant war with the United States. But they calculated that American mobilization would move too slowly to stop a German victory on the Western Front. That gamble helped push the United States into the war in April 1917. Once naval convoys were put into practice, sinkings fell short of the optimistic projections made by Germany's Imperial Admiralty Staff.

  British and Allied submarines ranged across the Baltic, North, Mediterranean, and Black Seas, as well as the Atlantic Ocean. Only a handful of actions took place outside that wider European and Atlantic theatre. One early German U-boat sinking counted as the first combat victory ever scored by a modern submarine. Another U-boat sank three British cruisers in under an hour, a feat that established the submarine as a serious new force in naval warfare. More than 5,000 Allied ships were sunk by U-boats over the course of the war.

  German submarines also laid naval mines and attacked ships carrying iron ore across the Baltic Sea. A British submarine flotilla operated in the Baltic in support of Russia until the Treaty of Brest-Litovsk ended that cooperation. During the war, Britain tried to build a submarine fast enough to operate alongside its battleship fleet, a design called the Fleet Submarine. Reaching the required 20 knots on the surface meant powering the resulting K-class boats with steam engines. In practice, the K-class proved a constant problem and could never operate effectively alongside the fleet it was built to support. The peace that followed would decide who was allowed to build submarines at all. That question would occupy treaty negotiators for the next two decades.

• 4. Wolf Packs in the Atlantic

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  France built submarine cruisers between the wars. Britain, meanwhile, armed submarines with battleship-caliber guns. Both nations, along with others, also experimented with submarines built to carry small reconnaissance aircraft, including the French vessel Surcouf. The Treaty of Versailles barred Germany from building submarines, though Germany built some in secret anyway. That arrangement went unrecognized until the Anglo-German Naval Agreement of 1935, when Britain accepted German submarine numbers on par with the Royal Navy.

  World War II split submarine combat mainly between the Atlantic and the Pacific. The Mediterranean Sea also saw heavy submarine activity, particularly for Britain and France, and for Germany as well. Italy took part as well, though its greatest successes came from midget submarines and human torpedoes rather than conventional boats. In the Atlantic, German U-boats again targeted Allied convoys, just as they had in the First World War. British submarines stayed active too, especially in the Mediterranean and off Norway, striking Axis warships, submarines, and merchant shipping.

  Hitler initially ordered his submarines to follow prize rules, but he lifted that restriction in December 1939. Mass submarine attacks had already appeared in the First World War, but the coordinated 'wolf pack' was primarily a Second World War U-boat tactic. The pattern worked like this: a number of U-boats fanned out across a convoy's likely path. A boat sighting a convoy would radio its course, speed, and composition back to German Naval Command. That submarine then shadowed the convoy, reporting any change in course, while the rest of the pack converged on its position. Once the pack had gathered, it attacked on the surface at night, withdrawing at dawn and resuming again at dusk. As warship and aircraft escorts multiplied, U-boat losses grew past what Germany could sustain. Many boats were lost, and with them the experienced commanders who had led the earliest attacks. Almost 3,000 Allied ships, 175 of them warships and 2,825 merchantmen, were sunk by U-boats over the course of the war. Germany's own U-boat fleet paid an even heavier price. It lost 793 boats and about 28,000 of its 41,000 submariners, a casualty rate near 70 percent. While the Atlantic ground down Germany's U-boat crews, a very different submarine campaign was unfolding elsewhere. On the other side of the world, it was just beginning to turn against Japan.

• 5. The Pacific and Other Theatres

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  By the war's end, United States submarines had destroyed over half of all Japanese merchant ships, sinking well over five million tons of shipping. British and Dutch submarines joined the fight against Japanese shipping too, working mostly in coastal waters. Dutch boats alone sank fifty enemy ships across the war, totaling roughly 200,000 tons. Japanese submarines opened the war successfully, sinking two United States fleet aircraft carriers, a cruiser, and several other ships. But Japan's doctrine focused on attacking warships rather than more vulnerable merchant vessels. That choice left its smaller fleet ineffective over the long run, and it suffered heavy losses to Allied anti-submarine efforts. Italian submarines, and one German submarine, also operated in the Pacific. Distance and strained relations with their Japanese ally kept them from becoming an important factor there.

  Japanese submarines also operated in the Indian Ocean, forcing Britain's surface fleet to withdraw to the east coast of Africa. Some German and Italian submarines operated there too, but never enough to play a significant role. The Pacific's war of attrition proved that submarines could strangle an island nation's supply lines. The next test of that idea would not come from a world war, but from a string of smaller, sharper conflicts.

• 6. A Fragile Peace

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  Since 1945, only a handful of conflicts have made real use of submarines. They include the Korean War, the Indo-Pakistani War of 1971, the Falklands War, and the 2026 Iran War. Submarine-launched land-attack missiles were later fired at targets in Iraq and Afghanistan. Apart from those exceptions, submarine warfare largely stopped after 1945, so strategic thinking about submarines has had to develop apart from real combat experience. With so little live combat to study, navies would have to reason their way toward the next era of underwater warfare almost entirely on paper.

• 7. The Nuclear Shift

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  The nuclear-powered submarine arrived in the 1950s and changed strategic thinking about submarine warfare almost overnight. These boats could run faster and dive deeper, with far longer endurance than their diesel predecessors. Their larger hulls also let them double as missile-launching platforms. Nuclear power gave submarines greater accuracy and let them use torpedoes against ships, other submarines, and land targets alike. In response, the attack submarine grew more important, particularly in its role as a hunter-killer. For a time, the United States also used nuclear submarines as radar pickets. Sensors and weapons both advanced substantially during this period as well.

  During the Cold War, the United States and the Soviet Union played what was called a 'cat-and-mouse' game. Each side worked to detect, and sometimes even trail, the other's submarines. As the odds of unrestricted submarine warfare declined, thinking about conventional submarines shifted toward their use against surface warships. A submarine's mere presence could be enough to limit a surface fleet's freedom to operate. Hunter submarines were developed in turn to counter that very threat. Submarine-launched autonomous unmanned vehicles have since extended the submarine's role even further. New air-independent propulsion methods have also reduced the need for diesel-electric submarines to surface, the moment when they are most vulnerable. Nuclear submarines, despite their far greater size, could already generate their own air and water, limiting their need to surface in the first place. That capacity to stay submerged indefinitely would go on to define how navies think about the submarines being built today.

• 8. Modern Fleets

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  Japan's Maritime Self-Defense Force has launched a new submarine model every few years amid today's more fractured geopolitical climate. South Korea has upgraded Germany's already capable Type 209 design and sold copies of its own version to Indonesia. Russia, meanwhile, has improved the old Soviet Kilo-class model into something strategic analysts describe as roughly on par with 1980s-era submarines.

  Military historian John Keegan makes a bold prediction in his book The Price of Admiralty. He argues that submarines will eventually take over almost every role currently held by surface warships. His reasoning is that submarines alone can evade the growing intelligence net of space satellites and aircraft. That net, he argues, is what an evenly matched fight between modern states would inevitably bring to bear.

  A 2024 study found that climate change may reduce how detectable submarines are in certain locations. Thinking about the submarine's importance has shifted toward an even more strategic role. That shift centers on the nuclear ballistic missile submarine, which carries warheads built for a guaranteed second strike. In March 2026, during Operation Epic Fury, the United States sank an Iranian frigate near Sri Lanka with a single MK 48 torpedo.