The Royal Navy, for its part, expended hundreds of expensive antisubmarine munitions and dispatched 2,253 helicopter sorties chasing false contacts—without detecting the San Luis on either occasion it closed within firing range.

The brief but bloody naval war that occurred in 1982 over the Falkland Islands, known as the Malvinas in Argentina, is typically viewed as a triumph of British naval power. A Royal Navy task force managed to beat off heavy air attacks to take back the South Atlantic archipelago from Argentine troops.

(This first appeared in 2016.)

For most of the war, a lone Argentine diesel submarine, the San Luis, opposed the Royal Navy at sea. Not only did the San Luis return home unscratched by the more than two hundred antisubmarine munitions fired by British warships and helicopter, but it twice ambushed antisubmarine frigates. Had the weapons functioned as intended, the British victory might have been bought at a much higher cost.

Argentina’s ruling military junta seized the disputed Falkland Islands opportunistically in order to score political points at home. Not expecting a real war, the junta miscalculated how quickly British prime minister Margaret Thatcher would escalate against their use of force with her own.

This lack of planning was manifest in the unpreparedness of the Argentine Navy’s submarine fleet. One was in such decrepit condition it could not safely submerge, while the more modern Salta was undergoing repairs. The older Santa Fe inserted frogmen to assist in the initial invasion on April 2. It was not until the following day that the most modern available sub, the San Luis, received orders at its dock at Mar de Plata to depart on a combat patrol around the area of the Malvinas.

The San Luis was a German Type 209 diesel submarine built in large numbers to serve as a smaller, cost-efficient submarine for less wealthy countries. Displacing just 1,200 tons with a crew complement of thirty-six, the San Luis carried fourteen Mark 37 antisubmarine torpedoes and ten German-manufactured SST-4 wire-guided torpedoes for use against surface targets. It could swim at forty-two kilometers an hour underwater or twenty-one on the surface, and had a maximum diving depth of five hundred meters.

It would be a cliché common to many tales of unlikely military accomplishments to emphasize the skill of the San Luis’s crew—but in fact, Argentina’s best submarine officers were in Germany at the time of the Falkland War. In their place, the San Luis made do with junior petty officers in charge of many keys departments of the ship. Its commander, Frigate Captain Fernando Azcueta, was a submarine veteran—but did not have much experience with the Type 209 model.

Moreover, the San Luis was in terrible condition and had to undergo rapid, incomplete repairs. Its snorkel was leaky, its bilge pumps were malfunctioning and one of the four diesel engines was not operational. Divers spent almost an entire week trying to clean crustaceans from the San Luis’s hull and propeller, which were impeding the vessel’s speed and stealth.

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The Argentine sub finally went to sea on April 11, and moved into a holding position while the political situation continued to deteriorate. Things did not come to a promising start. The San Luis’s fire control system allowed it to automatically guide three torpedoes simultaneously after launch. So, of course, it broke down after only eight days at sea, and none of its inexperienced petty officers knew how to fix it. They crew would only be able to launch one torpedo at a time under manual wire guidance. Still, it was decided the San Luis should proceed with its mission.

Meanwhile, the Santa Fe, an old Balao-class submarine that had served the U.S. Navy in World War II, was dispatched on April 17 to ferry marines and technicians to reinforce troops who had seized the island of South Georgia. Though it successfully deployed the troops on April 25, it failed to depart quickly enough and was detected at 9 a.m. by the radar of a British Wessex helicopter, which was soon joined by Wasp and Lynx helicopters. The Santa Fe was damaged by two depth charges, missed by a torpedo, struck by AS-12 antishipping missiles, and strafed with machine-gun fire. The captain beached the submarine, which was captured along with its crew by British troops shortly after. The attack on the Santa Fe marked the first shots of the British campaign.

The following day, the San Luis was ordered to sail for the waters around the disputed islands, and was authorized on the twenty-ninth to fire on any British warships it encountered.

However, the Royal Navy had intercepted the San Luis’s communications and deployed its helicopters and frigates to hunt it down. By one count, the Royal Navy had ten frigates or destroyers and a helicopter carrier assigned at least in part to antisubmarine duties, as well as six submarines on patrol.

On May 1, the San Luis’s passive sonar detected the HMS Brilliant and Yarmouth, both specialized antisubmarine frigates. Azcueta launched an SST-4 torpedo at a range of nine kilometers—but shortly after launch, the guidance wires on the torpedo cut out. Azcueta quickly dove his sub into hiding on the seabed. The Brilliant detected the attack, and the two frigates and their helicopters went into a frenzied pursuit of potential sonar contacts. Launching thirty depth charges and numerous torpedoes, the British vessels successfully blew up several whales for their efforts.

The following day, the British submarine Conqueror torpedoed the Argentine cruiser General Belgrano, which sank along with 323 members of its crew. The entire Argentine surface fleet subsequently withdrew to coastal waters, leaving the San Luis the only Argentine vessel opposing the British invasion force. British ships and helicopters began reporting sonar contacts and periscope sightings everywhere, and launched nine torpedoes in waters the San Luis never even ended up approaching.

The San Luis’s crew, for its part, thought they had been fired upon by a British submarine on May 8, and after taking evasive maneuvers, launched a Mark 37 torpedo against an undersea contact. The torpedo was heard to explode and the contact was lost. This, too, was likely a whale.

Two days later, San Luis detected the Type 21 antisubmarine frigates HMS Arrow and Alacrity on the northern passage of the Falkland Sound. Masked by the noise produced by the fast-moving frigates, the San Luis crept within five kilometers of the Alacrity, fired another SST-4 torpedo and readied a second for launch.

Yet again, the wires of the SST-4 cut out shortly after launch. However, some accounts state the torpedo actually struck a decoy being towed by HMS Arrow, but failed to detonate. Azcueta gave up on firing the second torpedo and ordered the San Luis to disengage to avoid a counterattack.

However, the British ships cruised on, unaware of the attack. The captain of the Alacrity did not even learn of the close call until after the war!

Demoralized, Azcueta radioed home that the torpedoes were useless, and he received permission to return to base, which he accomplished on May 19. The Argentine garrison surrendered on June 14 before the San Luis could be put back to sea. Fifteen years later, the San Luis became one of only three Type 209 submarines to be decommissioned after an incomplete overhaul. Another fifty-nine serve on in various navies.

What went wrong with the San Luis’s torpedoes? There are a half-dozen explanations, variously holding crew error and technical flaws culpable. Manufacturer AEG first claimed the torpedoes had been launched from too far away, and without active sonar contact. Another claim is that the Argentine crews mistakenly reversed the magnetic polarity of the gyros in the torpedoes, causing them to run astray. However, there is also evidence that the torpedoes failed to arm their warheads and could not maintain depth. Suggestively, AEG implemented numerous upgrades to the torpedo after the Falklands conflict.

The San Luis was no super-submarine, nor did it have a super-crew. Yet, benefiting from a competent commander using ordinary tactics, it still managed to run circles around a dozen antisubmarine frigates from one of the most capable navies in the world, and might easily have sunk several warships had its torpedoes functioned as intended.

The Royal Navy, for its part, expended hundreds of expensive antisubmarine munitions and dispatched 2,253 helicopter sorties chasing false contacts—without detecting the San Luis on either occasion it closed within firing range.

Real submarine warfare has been, thankfully, extremely rare since World War II. The Falkland experience suggests that cheap diesel submarines could be very difficult to counter even when facing well-trained and well-equipped adversaries.

Sébastien Roblin holds a Master’s Degree in Conflict Resolution from Georgetown University and served as a university instructor for the Peace Corps in China. He has also worked in education, editing, and refugee resettlement in France and the United States. He currently writes on security and military history for War Is Boring.