After receiving word of the Italian declaration of war on the evening of May 23, 1915, Admiral Haus sailed from Pola with the largest Austro-Hungarian naval force to put to sea during the war. His plan was to attack the Italian east coast, hoping to disrupt Italian troop movements to the front and to inflict an early blow against Italian morale. Haus’ bombardment force consisted of the 12 dreadnoughts and battleships, the armored cruiser Sankt Georg, the cruiser Novara, five destroyers, and 29 torpedo boats. Haus conducted a carefully planned attack on a number of targets in this opening engagement and dispatched several strike groups from the main bombardment fleet. Zrinyi and two torpedo boats bombarded the railway facilities at Senigallia, temporarily severing the coastal rail line; Radetzky and two torpedo boats attacked the railway near the mouth of the Potenza River; Sankt Georg and two torpedo boats bombarded Rimini; finally Novara, a destroyer, and four torpedo boats attacked the Porto Corsini naval base near Ravenna. The rest of the bombardment force, including Viribus Unitis, Tegetthoff, Prinz Eugen, Erzherzog Franz Ferdinand, Erzherzog Karl, Erzherzog Ferdinand Max, Erzherzog Friedrich, Habsburg, Arpad, Babenberg, four destroyers, and 20 torpedo boats, targeted the port and military facilities at Ancona. The bombardments went off without a hitch and all of Haus’ battleships were back in Pola before midday on May 24. Here Babenberg opens fire on the the Cantiere Liguro Anconitano shipyard in Ancona.
The last major operation involving Austro-Hungarian battleships in World War I has its roots in a naval mutiny that broke out aboard several of the larger ships stationed at Cattaro on February 1, 1918. On February 3 the three Erzherzog-class battleships sailed into the Bocche and demanded the surrender of the mutineers, who were also threatened with bombardment from the coastal batteries and other ships in the port that had remained loyal. The mutiny ended with over 800 sailors dismissed from the service. Some of the blame for the poor morale throughout the fleet due to months of inaction fell upon Flottenkommandant Admiral Maximilian Njegovan, who was relieved of duty after the mutiny. Linienschiffskapitän Miklos Horthy de Nagybanya was selected personally by Kaiser Karl I to replace Njegovan, primarily because of his proven aggressive spirit. Horthy, commander of the cruiser Novara, had led a successful cruiser raid against the Otranto Barrage in May 1917. If anyone could instill an aggressive and driving spirit into the fleet, Kaiser Karl believed that Horthy was the man for the job.
Horthy had an arduous task before him. Not only did he have to address the poor morale through the navy but he also had a smaller battle fleet to command than had his predecessors. Budapest was in shipyard hands, having a Skoda 380mm L/17 howitzer mounted in place of her forward main turret. Monarch and all of the Habsburg-class battleships had been decommissioned, freeing their crews for use in the growing U-boat and naval aviation arms. The ships of the Erzherzog Karl class had been assigned to Cattaro as guard ships to take the armored cruisers that were decommissioned in the wake of the mutiny. This left the ships of the Radetzky and Tegetthoff classes as the core of the battle fleet stationed at Pola. It took Horthy some time to become acquainted with managing fleet operations so nothing large scale was immediately planned, but the new commander felt more action was critical to raise morale throughout the fleet. By late spring 1918 Horthy was ready to undertake an aggressive action with his capital ships, with the same élan with which he had led his cruisers in battle the previous year. This would be the first major operation for the fleet’s battleships since the bombardment of Ancona in 1915. On June 8, 1918 Horthy steamed out of Pola with Viribus Unitis and Prinz Eugen, followed a day later by Szent Istvan and Tegetthoff. After rendezvousing with the Erzherzog Karls out of Cattaro, Horthy planned to lay in ambush with his battleships north of the Otranto Barrage after several of his cruisers and destroyers made a hit-and-run raid against the line and enemy shipping. He hoped to welcome pursuing Allied cruisers into the waiting embrace of his battleships.
On the opposite side of the Adriatic in the early evening hours of the 9th, the Italian torpedo boats 15 OS and 18 OS left Ancona, each towing an MAS boat, MAS 15 and MAS 21. The MAS boats had orders to search throughout the night for targets off the Dalmatian coast along the main Austro-Hungarian supply route between Fiume and Cattaro. In command of the MAS boats was Lt. Luigi Rizzo, who was eager to see action. The evening patrol passed without incident. Shortly before 0300 on the 10th, Rizzo ordered his MAS boats to proceed to the rendezvous point with the torpedo boats. At 0315 Rizzo spotted a rising cloud of smoke astern and ordered his boats to reverse course to investigate. What he found greatly excited him. Rounding Sansego Island at a leisurely 14 knots were two Austro-Hungarian dreadnoughts and their escorts. Rizzo, aboard MAS 15, ordered full throttle and the boats headed for the dreadnoughts. At 0325 the MAS boats sped through the Austro- Hungarian escort screen and fired their torpedoes. Aboard Szent Istvan the sea seemed calm; visibility was good except to the west, where the view was hampered by a slight haze. At 0330 a muffled explosion was felt along the starboard amidships of the dreadnought, followed by a second only moments later. The lookouts quickly scanned the horizon for enemy vessels but nothing could be seen. They then saw torpedo boat TB 76 altering course and firing shots from its bow gun. Beyond the water spouts of TB 76’s shots they spotted two MAS boats speeding off in the distance. The Szent Istvan had been struck by two torpedoes from Rizzo’s MAS 15. Within minutes the MAS boats dropped depth charges to ward off TB 76 and soon accelerated out of range of the torpedo boat’s guns. Rizzo had just earned himself a second medaglia d’oro al valor militare.
Shortly after the torpedoes struck, Linienschiffskapitän Heinrich Seitz ordered the ship’s turbines stopped. The aft boiler room had flooded and water began to leak into the forward boiler room after the explosions had ruptured the main bulkhead between the two rooms. A list of 10º to starboard developed but was brought back to 7º through counter-flooding. Captain Seitz signaled the Tegetthoff to prepare to take the crippled Szent Istvan in tow, but Tegetthoff had broken away at full speed on a zigzag course. The torpedoes fired from MAS 21 missed the dreadnought, causing her captain to order evasive action. There were several false sightings of periscopes by the jittery lookouts, followed by shots fired into the empty sea. This frantic search for phantom submarines went on for over an hour. After counter-flooding, Captain Seitz ordered Szent Istvan’s turbines back on and, creeping along at 4.5 knots, set a course for Brgulje, on the coast. In the lower decks the damage control teams were having difficulties containing the flooding. Water began leaking into the forward boiler room and adjacent ammunition stores. Bulkheads were failing to hold the force of the water to the extent that rivets were beginning to shoot out of their holes. Attempts to shore up the leaks with collision mats and hammocks failed and eventually the forward boiler room flooded. Soon all of the boilers were out except for two on the port side and these did not generate enough power to keep the pumps working. Again the dreadnought came to a halt. As a result of ineffective underwater protection, poorly constructed bulkheads (most likely due to the Danubius yard’s inexperience in building large warships), and the loss of the pumps, water continued to spread to other compartments.
At 0520, when Tegetthoff had returned from its wild goose chase, Szent Istvan signaled its sister ship to arrange a tow as quickly as possible. Her list to starboard continued to increase and eventually the casemated secondary batteries slipped below the waterline. The crews worked furiously to prepare a tow but at 0538 Szent Istvan began to lurch to starboard and the tow line had to be cut. The order to abandon ship was given as Tegetthoff backed away. The crew lined up in an orderly fashion on the sloping deck, some jumping into the water. At 0605 Szent Istvan made its final death roll and capsized. Captain Seitz and the ship’s senior officers were thrown off the bridge and a number of sailors scurried on to the keel as the ship rolled over, many injuring themselves on the sharp barnacles on the hull. At 0612 the dreadnought slipped beneath the waves. Four officers and 85 sailors were killed by the explosions or went down with the ship and the rest of the ship’s crew were rescued by Tegetthoff and the escorts. When word of the attack on Szent Istvan reached Admiral Horthy, he decided to abandon the operation against the Otranto Barrage, the element of surprise having been lost. Not knowing that Rizzo had happened upon Szent Istvan by pure chance, Horthy was convinced at the time that the Italians knew of his movements and believed that they had dispatched submarines and MAS boats to attack his forces. By the morning of June 11, Horthy’s remaining dreadnoughts had safely returned to Pola. Thus ended the last major fleet action conducted by the k. u. k. Kriegsmarine.