On the 28th of July, the Austro-Hungarian Empire initiated hostilities against the Kingdom of Serbia, marking the commencement of what would later be referred to as "The Great War." In the years leading up to this conflict, the relationship between the Italian Kingdom and the Austro-Hungarian Empire had undergone some degree of normalization. In 1912, the Italian Kingdom even joined the Triple Alliance, leading the Monarchy's admiralty to envision an ambitious strategy for its navy.
Given Italy's status as an ally, their strategic calculations involved maintaining a minimal naval presence in their ports for the defence of their home bases while primarily deploying their fleet in significant combat operations throughout the Mediterranean. This plan hinged significantly on the availability of coal stored in Italian ports to support collaborative efforts with the Regia Marina.
However, the outbreak of the war brought a swift change in circumstances. Italy declared its neutrality in the conflict on the 1st of August, effectively shattering Austro-Hungarian aspirations for implementing their naval strategy.
Tegetthoff didn't have to wait long for his first call to action. The German Mediterranean Division, consisting of the battlecruiser SMS Goeben and the light cruiser SMS Breslau, sought assistance from the Austro-Hungarian fleet. These German vessels were attempting to escape Messina, where they had been refuelling just before the outbreak of war. As the first week of August arrived, British ships began to assemble off Messina in an attempt to corner the Germans.
While Austria-Hungary's fleet hadn't fully mobilized yet, a task force was quickly assembled to aid the German ships. This group included Tegetthoff, Viribus Unitis, Prinz Eugen, three Radetzkys, the armoured cruiser Sankt Georg, the scout cruiser Admiral Spaun, six destroyers, and 13 torpedo boats. However, the Austro-Hungarian high command, mindful of not provoking a conflict with Great Britain, instructed their fleet to avoid British vessels and openly support the Germans only while within Austro-Hungarian waters. On August 7, when the Germans successfully left Messina, the Austro-Hungarian fleet set sail for Brindisi, intending to rendezvous with the Germans and escort them to a friendly port in Austria-Hungary.
Yet, the German movement toward the Adriatic's mouth had been a diversion to confuse the British and French pursuit. The German ships, instead of heading towards the Black Sea, rounded the southern tip of Greece and headed for Turkey. In response, the Austro-Hungarian fleet opted to return to Pola rather than follow the Germans.
After France and Britain declared war on Austria-Hungary on August 11 and 12, the French Navy devised a strategy to seal off the entrance to the Adriatic Sea and engage any Austro-Hungarian vessels with their Anglo-French fleet. Admiral Lapeyrère decided to target the Austro-Hungarian ships blockading Montenegro. The resulting Battle of Antivari put an end to Austria-Hungary's blockade and firmly established control of the Adriatic Sea entrance in the hands of Britain and France.
At this juncture, the Austro-Hungarian Navy essentially served as a "fleet at being," meaning that its mere existence compelled the British and French Navies to allocate a substantial force to protect the Adriatic entrance. Meanwhile, the Austro-Hungarian Navy anticipated Italy switching sides, prompting Tegetthoff and her sister ships to remain docked in port.
On May 4, Italy formally terminated its alliance with Germany and Austria-Hungary, providing advance notice to the Austro-Hungarians of Italy's impending hostilities against them. Haus made arrangements for Tegetthoff and her sister ships to be ready for a massive strike in the Adriatic as soon as war was officially declared. On May 23, 1915, approximately two to four hours following the Italian declaration of war reaching the primary Austro-Hungarian naval base at Pola, the Austro-Hungarian fleet, which included Tegetthoff, set sail to bombard the Italian coastline.
While some ships targeted secondary objectives and others were positioned to the south to monitor any Italian vessels potentially moving north from Taranto, the core of the Austro-Hungarian Navy, led by Tegetthoff and her sister ships, headed towards Ancona. The bombardment of the Ancona region proved to be a significant success for the Austro-Hungarian Navy. In the port of Ancona, an Italian steamer was obliterated, and three others sustained damage. The port's infrastructure and the adjacent towns suffered extensive damage. Rail yards, port facilities, and local shore batteries were either damaged or demolished. Multiple docks, warehouses, oil storage tanks, radio stations, and fuel depots were set ablaze by the bombardment, while the city's electricity, gas, and telephone lines were severed. Inside the city, Ancona's police headquarters, military barracks, a military hospital, a sugar refinery, and the offices of the Bank of Italy all suffered harm. The attack resulted in the loss of 30 Italian soldiers and 38 civilians, with an additional 150 people wounded.
Subsequently, the Austro-Hungarian Navy proceeded to bombard the Montenegrin coast without facing opposition; by the time Italian ships arrived, the Austro-Hungarian fleet had safely returned to Pola. The primary aim of the Ancona bombardment was to impede the Italian Army's deployment along the Austria-Hungary border by disrupting crucial transportation systems. This surprise attack on Ancona successfully delayed the Italian deployment to the Alps by two weeks. This delay provided Austria-Hungary with valuable time to fortify its Italian border and redeploy troops from the Eastern and Balkan fronts, while also delivering a severe blow to Italian military and public morale.
In the following years, Tegetthoff and her sibling ships experienced a period of relative inactivity. The Austro-Hungarian navy faced challenges such as coal shortages, concerns about Adriatic sea mines, and the admiralty's reluctance to risk their dreadnoughts in direct engagement with the enemy fleet, as the potential loss was a grave concern. Consequently, they rarely ventured out of port, except for occasional gunnery practice.
In 1918, a new admiral assumed command of the Navy, Miklos Horthy. Horthy utilized his appointment as an opportunity to regularly take the Austro-Hungarian fleet out of port for maneuvers and gunnery exercises. These operations marked the most significant naval activities Tegetthoff had witnessed since the war's outbreak. The purpose of these gunnery drills and maneuvers extended beyond addressing issues stemming from several mutinies; they were also part of preparations for a major offensive.
Horthy's strategic approach differed from that of his predecessors, and shortly after taking charge of the navy, he made the decision to embark on a substantial fleet action. His goal was to combat low morale and boredom and facilitate the breakout of Austro-Hungarian and German U-boats from the Adriatic into the Mediterranean. Following several months of training, Horthy concluded that the fleet was ready for a significant offensive in early June 1918. Horthy was determined to use the fleet to attack the Otranto Barrage, intending to replicate his earlier successful raid on the blockade in May 1917.
Horthy envisaged a massive assault on the Allied forces, with Tegetthoff and her three sister ships forming a substantial part of the offensive. They would be accompanied by the three ships of the Erzherzog Karl-class pre-dreadnoughts, the three Novara-class cruisers, the cruiser Admiral Spaun, four Tátra-class destroyers, and four torpedo boats. The operation would also involve submarines and aircraft to hunt down enemy ships on the flanks of the fleet.
Regrettably, his plan did not unfold as intended when, during the initial stages of the attack, a portion of his group, including SMS Tegetthoff and SMS Szent István, was detected early.
While en route to the harbor at Islana, located north of Ragusa, to rendezvous with Viribus Unitis and Prinz Eugen for a coordinated assault on the Otranto Barrage, Szent István and Tegetthoff endeavoured to achieve maximum speed to catch up with the rest of the fleet. However, in their haste, Szent István's turbines began to overheat, necessitating a reduction in the speed of both ships. When they attempted to generate more steam to increase their velocity, Szent István produced an excess of smoke. Around 3:15 am on June 10, two Italian MAS boats, namely MAS 15 and MAS 21, spotted the smoke emanating from the Austrian vessels while returning from their patrol along the Dalmatian coast. Both boats successfully infiltrated the escort screen and proceeded to engage each of the dreadnoughts.
MAS 21 targeted Tegetthoff, but its torpedoes missed the ship. At 3:25 am, MAS 15 launched its two torpedoes, successfully hitting Szent István followed the attack both boats managed to elude any pursuit. Tegetthoff, believing the torpedoes were fired by submarines, withdrew from the formation and initiated evasive zigzag manoeuvres to fend off further attacks, even firing on suspected submarine periscopes.
Meanwhile, Szent István sustained damage from two 45-centimeter torpedoes adjacent to her boiler rooms, and attempts to seal the breaches proved futile. Upon returning to the formation at 4:45 am, Tegetthoff endeavoured to tow Szent István, which ultimately proved unsuccessful. Realizing that Szent István was destined to sink, the crew of Tegetthoff emerged on her decks to salute the sinking ship. At 6:12 am, as the pumps proved inadequate to the task, Szent István capsized near Premuda. Horthy, perceiving that the element of surprise had dissipated, called off the attack. Subsequently, Tegetthoff and the rest of the fleet returned to the base at Pola, where they remained for the duration of the war.
Tegetthoff's final wartime engagement occurred on July 17, 1918, when Pola endured the most extensive air raid the city experienced during the conflict. Over 200 bombs were dropped by 66 Allied aircraft, yet Tegetthoff remained unscathed in the midst of the attack.
Following the armistice and the dissolution of the monarchy, the fate of the fleet remained uncertain and was a subject of dispute involving Italy and the newly established nations of Slovenia, Croatia, and Serbia. Ultimately, Tegetthoff raised the Italian flag and was escorted to Venice, where she was exhibited as a war trophy by the Italians. It wasn't until 1920 that the final allocation of the ships was determined by the Allied powers under the provisions of the Treaty of Saint-Germain-en-Laye, with Tegetthoff formally transferred to Italy.
During this period, she played a role in the film "Eroi di nostri mari" ("Heroes of Our Seas"), which portrayed the sinking of Szent István. Subsequent to the adoption of the Washington Naval Treaty in 1922, Tegetthoff was dismantled between 1924 and 1925 at La Spezia. After her disassembly, one of her anchors found a place on display at the Monument to Italian Sailors in Brindisi, where it remains to this day.