A weapon that was hard to take seriously
Almost from the start, the idea of a submarine was tied to attack. A craft that could approach an enemy ship unseen offered what looked like a cheap way to threaten an expensive one. Yet for much of the nineteenth century, navies saw little reason to invest. The boats were slow, hard to steer, dangerous to the crew and short of range, and the weapon they carried was often an explosive charge that the crew had to fix to a hull by hand.
Early attack attempts
The American Revolution produced the first well-known attempt, a one-person craft meant to place an explosive under a British warship in 1776. It did not succeed. During the American Civil War the Confederate H. L. Hunley sank the Union steam sloop USS Housatonic in February 1864 using an explosive on a spar, and it is generally described as the first submarine to sink a warship in combat. The Hunley did not return from the mission.
These episodes were dramatic but did not change naval planning. The weapons were crude, and the crews were at greater risk than their targets.
The torpedo changes the equation
The decisive improvement was the self-propelled torpedo, developed in the 1860s, a story associated with the engineer Robert Whitehead, though many people contributed. A submarine no longer had to touch its target. It could fire from a distance, and a torpedo hit below the waterline could do serious harm to even a large ship.
Around 1900 several navies, including the US, British, French and Japanese, began buying submarines in earnest. Opinion was divided. Some officers saw them as a useful coastal defense. Others, according to many accounts, considered attacking without warning to be unsporting or dishonorable, and doubted that such craft would matter. That debate faded in practice once the boats were in service.
The First World War
World War I showed that the doubters were wrong. In September 1914, the German submarine U-9 sank three British armored cruisers, Aboukir, Hogue and Cressy, in a single morning. Germany later used its U-boats against merchant shipping to cut Britain’s supply lines, which was a very different use from sinking warships.
The campaign was legally and morally contested. Under older rules, a warship was expected to stop a merchant vessel, search it and let the crew leave before sinking it. A submarine is too small and vulnerable to follow that procedure easily. The sinking of the passenger liner Lusitania in May 1915 caused outrage, and the policy of unrestricted attacks later became one of the factors in American entry to the war in 1917.
The Allies responded with depth charges, mines, patrols and, eventually, convoys, in which merchant ships sailed together under escort. Convoys are widely regarded as having reduced losses.
The Second World War
World War II repeated the contest on a larger scale, especially in the Atlantic. German U-boats operated alone and, later, in groups, while the Allies combined escorts, aircraft, improved sonar, radar and code-breaking. The campaign is often described as one of the longest continuous struggles of the war, and its outcome mattered for the supply of Britain.
Submarines were also used heavily in the Pacific, where American boats attacked Japanese shipping, and many submarines on all sides were lost. The casualty rate among crews was high compared with most other services.
Technically, most boats were still surface ships that could dive for limited periods. Late in the war, devices such as the snorkel let diesel-electric boats run their engines while just below the surface, and Germany introduced more streamlined designs that came too late to change the result.
Nuclear power and deterrence
After 1945, nuclear propulsion allowed a submarine to remain submerged for long periods. Combined with ballistic missiles, which began to be carried by submarines in the late 1950s and 1960s, this gave a few navies a new role: a hard-to-find force intended mainly to discourage a nuclear attack. The idea is that a submarine that cannot be easily located remains a credible deterrent. Whether that reasoning is sound is a matter of continuing policy debate, not settled fact.
Cold War attack submarines also tracked one another quietly, and much of what they did remains classified or only partly known.
Why “secret weapon” is partly a myth
Submarines were often described as secret weapons, but their value came less from secrecy of design than from concealment of position. Once navies learned to listen for them with hydrophones and sonar, the contest became about noise, detection and endurance. That is still the central issue in underwater warfare.