The Austro-Hungarian Navy's role in WWI can be compared to the German Navy's role in the same war. While it was smaller, it represented a serious threat to Allied forces in the Mediterranean. However, especially after Italy's entry to the war, it was outnumbered by the forces arrayed against it, and spent much of the war in port as a fleet-in-being. Lighter forces made a number of raids, though to limited effect, and their submarines fought a reasonably effective war against Allied shipping. The Austro-Hungarian Navy (k.u.k Kriegsmarine) also participated in a riverine campaign on the Danube against Serbia and Romania. As a quick note - most of the locations along the coasts of Croatia, Albania and Montenegro are referred to by their Italian names in the historical literature, so they are what I'm using.

The k.u.k Kriegsmarine entered the war with three dreadnought battleships, nine pre-dreadnoughts, ten cruisers and 25 destroyers, as well as five submarines and six obsolete battleships used as coast defence ships. One of the cruisers, the old protected cruiser Kaiserin Elisabeth, was at the German colony of Tsingtao in China, and would be scuttled there on the 2nd November 1915 before the colony surrendered. Over the course of the war, another dreadnought, three cruisers, five destroyers and 21 submarines would be added. The main Allied force responsible for countering it was the French Navy; on the 6th August 1914, the British and French agreed that France would have leadership of naval operations in the Mediterranean. The French fleet in the Mediterranean had four dreadnoughts, fifteen pre-dreadnoughts, fifteen cruisers and 20-40 destroyers, and would add three more dreadnoughts and up to 30 more destroyers. The Italian entry to the war would add another four dreadnoughts, twelve cruisers and 32 destroyers, with two dreadnoughts, ten cruisers and 20 destroyers entering service during the war. To this must also be added a fluctuating presence of British cruisers and destroyers. As such, it should be clear that the k.u.k Kriegsmarine was heavily outnumbered. As such, it remained confined mainly to the Adriatic Sea.

The first action in the Adriatic came on the 14th August 1914, three days after France declared war on Austria-Hungary. The French Admiral de Lapeyrere, having gathered his forces at Malta, swept northwards into the Adriatic, supported by two British armoured cruisers. The k.u.k Kriegsmarine had established, using its cruisers and destroyers, a blockade of Serbia and Montenegro. de Lapeyere was hoping to, by attacking these forces, draw out the main body of the Austrian fleet, and destroy it. He encountered little, but, off the Montenegrin town of Antivari, did find an Austrian cruiser, the Zenta and a destroyer. The Zenta attempted to fight to cover the escape of the destroyer, but was soon sunk. The remainder of the k.u.k Kriegsmarine did not engage. The remainder of 1914 saw no surface actions. The k.u.k Kriegsmarine made several bombardments of Allied troops in Montenegro, supporting the Austro-Hungarian advance. While the surface fleets were relatively idle, the submarine fleets were not. On the 21st December, the French dreadnought Jean Bart was torpedoed by the Austrian submarine U-12, while she was covering an Allied convoy to Montenegro, though she would ultimately survive. French submarines were not idle either, making several attempts to raid the main k.u.k Kriegsmarine base at Pola; the Curie was lost in one such attempt on the 20th December.

1915 followed a similar pattern, at least for the first few months; the k.u.k Kriegsmarine carrying out operations along the Balkan littoral, with the Allies doing little. There were several submarine engagements during this time; the French cruiser Léon Gambetta was sunk by U-5 in the most successful. The Italian entry to the war on the 23rd May changed things massively. The k.u.k Kriegsmarine immediately began operations against the Italian coastline. On the 23rd, the port of Ancona was bombarded by two destroyers, while another destroyer fought a short but inconsequential action with the Italian Turbine. The next day, the main battlefleet arrived; Ancona, and a number of towns nearby were bombarded by the battleships. Rimini was bombarded by the cruiser Sankt Georg, while Turbine was sunk by another Austrian cruiser, the Helgoland, and four destroyers. On the 5th, the Allies responded by bombarding a number of Austrian targets. Such operations continued through the summer of 1915. On the 28th July, the Austrians attempted a landing on the island of Pelagosa. While the troops were ultimately beaten back, a bombardment on the 17th August forced the Italian garrison to withdraw. Submarine operations also continued. On 9th June, the British cruiser Dublin was damaged by U-4, which would sink the Italian Giuseppe Garibaldi a month later. The Italians lost the submarine Medusa and destroyer Serpente to U-11 and torpedo boat PN-5 to U-10. The Allies, worried by the ability of Austrian and German submarines operating from the Adriatic to strike shipping routes in the Mediterranean, formed what is called the Otranto Barrage in September. Modelled on the Dover Barrage, this combined minefields with surface patrols and patrols of drifters (small fishing vessels) with indicator nets. The barrage was relatively unsuccessful at preventing submarines leaving the Adriatic, and provided targets for the k.u.k Kriegsmarine to strike against. In December, the Austrian campaign in Serbia came to a head. The Serbs withdrew into Albania, supported by Allied supplies through the port of Durazzo. On the 28th, the cruiser Helgoland, supported by five destroyers, attempted to raid this line of communication. While they sank the French submarine Monge, two destroyers struck mines. One sank soon after, and a third was damaged in attempting to recover the other, as a tow rope fouled her propeller. A force of Allied cruisers was thus able to intercept them, though the Austrians managed to escape, losing only the two mined destroyers.

1916 saw more raids, and more submarine operations. On the 6th February, the Helgoland made another inconclusive raid against Durazzo - this time, to strike against the Allied evacuation of the Serbian Army from the port. More raids were made against the drifters on the Otranto Barrage. During the night of the 22nd/23rd December, one such raid provoked a response by two Allied destroyer flotillas, one Italian and one French; in the ensuing confusion, the two flotillas became intermingled, and there were two accidental collisions. Allied and Austrian submarine operations continued. In one notable milestone, the French submarine Foucault was sunk by two Austrian seaplanes on the 15th September, becoming the first submarine to be sunk by aircraft.

1917 followed the same pattern. Austrian destroyers made a number of probes against the Otranto Barrage in March-May 1917. This preceded a raid by three cruisers and two destroyers on the 14th May. The destroyers encountered an Italian convoy, sinking the destroyer Borea and a merchant. The cruisers struck against the barrage, sinking 14 drifters and heavily damaged three more. Skipper Joseph Watt of the Gowan Lea would receive the Victoria Cross for his gallantry in opposing these cruisers with his ship's single 57mm gun. The Allies responded quickly, deploying a number of cruisers and destroyers; while they heavily damaged the Novara, reinforcements managed to see off the Allied cruisers before they could capitalise on this. As the Allies withdrew, HMS Dartmouth was torpedoed by UC-25 and nearly sunk, while the destroyer Boutefeu was mined while coming to assist. In December, an Italian motor torpedo boat would infiltrate Trieste harbour, sinking the old battleship Wien.

On February 1st 1918, there was a major mutiny at the naval base of Cattaro. The main grievance of the mutineers was poor food and poor treatment by the officers; however, it would soon expand to demands for peace, and to some extent, socialist policies. Support for the mutiny was not widespread, and did not spread to the army - there was a small exchange of fire between a military coastal defence unit and one of the mutinying ships, and between another mutinying ship and a loyalist one. The mutiny was swiftly put down, and 800 men charged; four would be executed for mutiny. On the 22nd April, a short action was fought between British and Austrian destroyers in the Otranto Straits, with little damage on either side. On the 9th June, another raid against the straits was attempted. This time, it would include the Austrian dreadnoughts, which were to cover the attacking cruisers. As they headed south, two Italian motor torpedo boats engaged them. MAS 15 put two torpedoes into Szent Istvan, while MAS 21 likely hit her with one. She rolled over and sank, taking 89 men with her. The raid was cancelled. On the 2nd October, an Allied force, including British, American, French and Italian warships, attacked Durazzo. This aimed to cover the flanks of the Allied advance from the Salonica beachhead. The port was bombarded, and a number of merchants and Austrian warships sunk or damaged. A month later, Italian divers, deployed by a primitive human torpedo, laid limpet mines on the hull of the dreadnought Viribus Unitis, sinking it. Unfortunately, the day before, the k.u.k Kriegsmarine had ceased to exist, and much of the fleet had been handed over to the State of Slovenes, Croats and Serbs, a short-lived predecessor to Yugoslavia.