In the same month that the U-9 was retired to training duties there began a cruise which was to make the name of Otto Hersing famous in Germany and well known even among the Allies. On April 25, 1915, Hersing in his submarine the U-21 sailed from Wilhelmshaven and, rounding the north of Scotland, made his way south to Gibraltar where he slipped unnoticed through the strait and entered the Mediterranean Sea.
His mission was to assist Germany's ally, Turkey, at the eastern end of the Mediterranean, where British ships and British Empire troops were attempting to force an opening through the key passage between the Aegean Sea and the Sea of Marmara known as the Dardanelles. If the British attempt were successful, the Allies would be able to ship supplies from Western Europe through the Mediterranean Sea and the Dardanelles to Russian ports on the Black Sea. Such a route would be of signal importance to the war effort of the Allies, and it was equally important to Germany and Turkey to frustrate British attempts to open it.
As a measure of support for Turkey against the massed might of Britain, Germany dispatched Hersing and his lone submarine, the U-21. Never before in its short history had a submarine attempted such a long cruise — 2,400 miles. The distance exceeded the U-21's normal cruising range, and there would be no friendly ports along the way in which Hersing could refuel.
Arrangements were made, though, for the U-21 to refuel at sea from the Hamburg-American steamer Marsala off the coast of Spain. Meeting the Marsala as planned, the U-21 took on twelve tons of diesel oil and two tons of lubricants, but Hersing soon found that the fuel from the steamer did not meet the needs of his submarine's engines. He had to try to reach his destination of Cattaro, an Austrian port on the Adriatic Sea, on what remained of the fuel he had taken with him from Wilhelmshaven.
On May 13, with its fuel tanks virtually empty of usable fuel, the U-21 was met by an Austrian destroyer and towed safely to its destination. On his arrival at Cattaro, Hersing learned of the bitter fighting taking place on the Gallipoli Peninsula.
The Allies had established a foothold on the Gallipoli shore, and the embattled Turkish troops were slowly losing ground. Supporting the Allies were the big guns of the British battleships Triumph and Majestic which could lob 12- and 14-inch shells into the battered Turkish lines from anchorages safely out of range of the smaller Turkish artillery.
After a week at Cattaro the U-21 put to sea again with the waters off the Gallipoli battleground as its destination. It arrived on May 26, 1915, and in the morning light Hersing saw through his periscope the dry hills of the peninsula, burned tan by the blazing Turkish sun. In these hills hundreds of thousands of troops grappled in combat.
In the waters off shore were ships of the Royal Navy. Hersing could see several large ones, and among them a single giant stood out. Submerging below periscope depth he set a dead-reckoning course toward it. In four and a half hours he had worked into position for attack. Through his periscope he could see that the ship was only 300 yards away. He could easily read the name on its stern: HMS Triumph.
Hersing quickly adjusted his position and fired a torpedo. Fascinated by the possibility of sinking so marvelous a target, Hersing could not bring himself to submerge beneath periscope depth. Through his eyepiece he soon saw the telltale froth of his torpedo's wake, and in moments there was a tremendous explosion against the mighty Triumph's hull.
The battleship, shaken to its keel, began to sink immediately.
Protecting destroyers saw the U-21's periscope and sped to the attack. To escape them, Hersing daringly passed directly beneath the torpedoed Triumph with its 11,000 tons sinking over his head.
The sight of the Triumph torpedoed and sinking was such an unexpected and appalling sight that even the instinct of self-preservation was momentarily forgotten by soldiers in opposing trenches. Both Ally and Turk stood up for several minutes in full view of each other to watch as the massive battleship began its plunge.
Returning the next day, Hersing attacked and sank the 15,000-ton Majestic . It capsized in only 50 feet of water, and for years its rusting bottom could be seen above the waters off the Gallipoli shore.
With two torpedoes Hersing had sunk two battleships in two days. A British analyst wrote that
"it may fairly be said that the appearance of U-boats on the scene marked the turning point in the Gallipoli operation"
— but when those words were written in 1917, the British did not yet know that it was not submarines but a single submarine that had changed the course of the battle in a way which Britain was never able to reverse.
A prominent military analyst who was a junior officer with the Allied force at the time particularly remembered the loss of the Triumph.
"All of us doughfeet [infantrymen] loved that ship. She was smaller than most British battleships, she could come closer inshore to give us fire support, and she was one of the finest gunnery ships in the Royal Navy. Time and again she came thundering to our help when we needed her most. We couldn't believe she was gone: in fact, we started a subscription of a month's pay all around to have her raised and put back into servicel But the real point is that with the loss of two battleships, the top brass decided that it was too risky to use battleships for close support of infantry, and after that the Gallipoli campaign didn't go at all well for our side."
Britain left the matter of artillery support partly to its own submarines in the area, which were able to surface, fire, and submerge again without fear of submarine attack, but whose light deck guns were feeble in comparison to the punishing armament of the now-sunk battleships.
In the end the Gallipoli campaign was a colossal failure and the British withdrew, having suffered a hundred thousand casualties. The Mediterranean-Black Sea supply route to Russia remained closed to the Allies.