Here's What You Need to Remember: The most successful anti-submarine ships were agile “torpedo-boat destroyers,” which sank U-Boats using deck guns and even ramming. Starting in 1916, Royal Navy vessels carried depth charge designed to detonate underwater, rupturing a submarine’s hull. These proved effective if the ship captains could guess the sub’s position. Statistically, naval mines proved deadliest, accounting for one-third of U-Boat losses.

When Congress voted on April 6, 1917 to declare war on Imperial Germany, the task before the U.S. Navy was clear: it needed to transport and supply over a million men across the Atlantic despite the Imperial German Navy’s ferocious U-Boat campaign, which reached its peak that month, sinking over 874,000 tons of shipping.

Indeed, Germany’s decision to recommence unrestricted submarine warfare in February was one of the decisive factors driving the United States, and later Brazil, into finally joining “the war to end all wars.”

While World War I submarines could only remain submerged for brief periods, they were highly successful at picking off unescorted merchants ship in the Atlantic and Mediterranean. Neither active sonar nor radar yet existed with which to track submarines, though the British had begun using hydrophones to listen for the noise of a submarine’s diesel engine.

The most successful anti-submarine ships were agile “torpedo-boat destroyers,” which sank U-Boats using deck guns and even ramming. Starting in 1916, Royal Navy vessels carried depth charge designed to detonate underwater, rupturing a submarine’s hull. These proved effective if the ship captains could guess the sub’s position. Statistically, naval mines proved deadliest, accounting for one-third of U-Boat losses.

For years, the Royal Navy resisted instituting a convoy system to guard merchant ships, preferring not to divert warships from offensive missions and believing the decrease in throughput from adhering to a convoy schedule would prove worse than the losses inflicted by U-Boats.

But that April, U-Boats had sunk one-quarter of all merchant ships bound for the UK, leaving it with just six week’s grain supply. Threatened with economic collapse, the Royal Navy finally instituted the convoy system. But the Brits had a problem: they could divert only forty-three out of the seventy-five destroyers required to escort convoys.

Naval liaison Rear Admiral William Sims convinced the navy to dispatch thirty-five U.S. destroyers to bases at Queenstown (modern-day Cobh), Ireland to fill in the gap. These began escorting convoys on May 24, usually supported by navy cruisers. In 1918, an even larger escort flotilla began operating out of Brest, France.

The U.S. Navy itself began the war with only fifty-one destroyers. It immediately faced a classic military procurement problem: politicians and admirals wanted to build more expensive battleships and battlecruisers, construction of sixteen of which had been authorized by the Naval Act of 1916.

But the Royal Navy already had the German High Seas fleet effectively bottled up in port with its larger force. While five coal-burning and three oil-burning U.S. battleships did join the blockade in 1918, they never saw action. Common sense prevailed, and battleship construction was halted in favor of building 266 destroyers.

More rapidly, the Navy commissioned hundreds of small 70-ton wooden-hulled “sub-chasers” equipped with hydrophones, 3” deck guns and depth charges. Civilian yachts were similarly converted. The Navy’s eleven L-class and K-class submarines were also deployed to Berehaven (now Castletownbere), Ireland and the Azores respectively to hunt (surfaced) U-Boats, but none encountered enemy forces during the war.

Hundreds of twin-engine HS maritime patrol planes were also procured to scour the seas for submarines. Though the seaplanes sank few if any submarines, they disrupted numerous attacks by forcing U-Boats to dive and abort their torpedo runs.

The convoy system proved a dramatic success, cutting shipping losses to less than half their peak. U-Boats simply lacked unprotected targets and were more likely to be lost combating escorts. Shipping losses gradually fell to roughly 300,000 tons per month, while U-Boat losses increased from three per month to between five and ten.

However, submariner-hunting remained a dangerous business in which a hunter could swiftly become hunted. On Nov. 17, 1917, the destroyer USS Cassin was pursuing U-61 near Ireland when the U-Boat counterattacked. Spotting a torpedo rushing towards the depth-charge launcher on the ship’s stern, Gunner’s Mate Osmond Ingram lunged forth to jettison the explosive charges but was caught in the blast that tore away the destroyer’s rudder. The Cassin remained afloat and shelled U-61’s conning tower, causing her to disengage. Ingram was posthumously awarded the Medal of Honor.

The destroyer Jacob Jones was not so fortunate when she was struck in the rudder by a torpedo fired by U-353 near Brest on December 6. Sixty-six crew perished abandoning ship as her depth charges detonated. Gallantly, U-Boat captain Karl Rose rescued two of the crew and radioed the position of the other survivors.

U.S. sub-hunters did score some successes. On November 17, the destroyers Fanning and Nicholson forced U-58 to the surface with depth charges, then engaged her with deck guns until her crew scuttled her. The converted yacht Christabel crippled a U-Boat with depth charges in May 1918 off the coast of Spain.

That month, the Imperial Navy began dispatching long-range U-Boat “cruisers” with huge 150-millimeter deck guns to maraud the U.S. coast. These sank ninety-three vessels, mostly small civilian fishing boats. The Germans hoped this would spread panic, causing the Americans to withdraw assets in Europe for home defense.

Notably, on July 18 the boat U-156 surfaced off the coastal town of Orleans on Cape Cod, Massachusetts, and proceeded to destroy a tugboat, four barges and the nearby shoreline with its cannons. Nine Coast Guard HS and Model R-9 seaplane bombers scrambled from NAS Chatham and peppered the withdrawing U-boat with bombs—none of which exploded.

The following day, the armored cruiser USS San Diego struck a mine probably lain by U-156 south of Long Island. The explosion flooded her engine room, causing the cruiser to sink with the loss of six hands—becoming the only capital ship lost by the navy. U-156 proceeded to sink twenty-one fishing boats in the Gulf of Maine, and even commandeered a trawler to assist in its rampage. But though the navy instituted coastal convoys, it didn’t withdraw ships from Europe.

U-Boats were also active in the Mediterranean, and Gibraltar-based American subchasers—often little more than civilian yachts fitted with 3” guns and depth charges—twice clashed with them, sinking at least one.

Perhaps the Navy’s most swashbuckling episode of the war occurred on October 2, 1918, when twelve U.S. subchasers covered an Italian and British surface force raiding the Albanian port of Durazzo. Dodging shells from shore batteries, the subchasers cleared a path through the defensive minefield for the accompanying capital ships. They then hounded away the submarines U-29 and U-31, heavily damaging both.

The navy’s deadliest anti-submarine measure was the North Sea Mine Barrage, a 230-mile-long chain of 100,000 naval mines between the Orkney islands and Norway. U-Boats seeking passage to the Atlantic had to wend through eighteen rows of Mark 6 mines concealed at depths of twenty-four, forty-nine and seventy-three meters deep, strung together with piano wire. Each of the horned steel spheres contained three hundred pounds of TNT. The barrage cost $40 million ($722 million in 2018 dollars) and required the deployment of eight large steamships. However, it sank between four and eight U-Boats—including the infamous U-156—and damaged another eight.

Ultimately, 178 out of 360 operational U-Boats were sunk during World War I. In return, the German subs sank 5,000 merchant ships totaling 12.8 million tons, killing 15,000 mariners. The U.S. Navy lost 431 personnel and five ships—its worst loss occurred when the collier USS Cyclops vanished with 306 crew in the Bermuda Triangle.

Despite its unglamorous duties, the U.S. Navy learned valuable lessons in the Great War about employing convoys, smaller submarine-hunters and maritime patrol planes that would save many lives in the even more destructive conflict that followed two decades later.

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. This first appeared in December 2018.