SM U-1 or U-I was the lead boat of the U-1-class of submarines or U-boats built for and operated by the Austro-Hungarian Navy (German: kaiserliche und königliche Kriegsmarine or k.u.k.

Facing defeat in October 1918, the Austro-Hungarian government transferred its navy to the newly formed State of Slovenes, Croats and Serbs to avoid having to hand its ships over to the Allied Powers.

Following the Armistice of Villa Giusti in November 1918, U-1 was seized by Italian forces and subsequently granted to the Kingdom of Italy under the Treaty of Saint-Germain-en-Laye in 1920.

[4] After observing the MTK design submitted in early 1905, the Naval Section of the War Ministry (German: Marinesektion) remained skeptical.

[6] After being recommended by Popper, Lake traveled to Austria-Hungary in 1906 to negotiate the details of his contract with the Navy, which formally ordered plans for the building of two boats—including U-1—on 24 November.

U-1 and her sister ship were described by the naval historians David Dickson, Vincent O'Hara, and Richard Worth as "obsolete and unreliable when completed and suffered from problems even after modifications".

[10] John Poluhowich writes in his book Argonaut: The Submarine Legacy of Simon Lake that U-1 was "completed to the satisfaction of Austrian officials".

[13][8] Following the onset of World War I, however, it became clear that Austro-Hungarian U-boats were best suited for offensive operations, namely raiding Allied shipping in the Adriatic and Mediterranean Seas.

The diving chamber was intended for manned underwater missions such as destroying ships with explosives and severing off-shore telegraph cables,[15] as well as for exiting or entering the submarine during an emergency.

This diving chamber ultimately proved its usefulness during the sea trials of U-1 and her sister ship when the crew of one submarine forgot to bring their lunches on-board before conducting an underwater endurance test.

The design also placed the diving tanks above the waterline of the cylindrical hull, which necessitated a heavy ballast keel for vertical stability.

[15][19][20][b] Indeed, the engine problems for U-1 were so significant that on multiple occasions her crew had to conduct emergency resurfacing to bring fresh air into the ship.

By the end of November 1912, the threat of conflict between Austria-Hungary, Italy, Serbia, and Russia, coupled with allegations of Serbian mistreatment of the Austro-Hungarian consul in Prisrena led to a war scare in the Balkans.

U-1 and her sister ship U-2 were both ordered to join the rest of the Austro-Hungarian fleet assembling in the Aegean Sea in the event of a war with Serbia and Russia.

[22] By December 1912, the Austro-Hungarian Navy had, in addition to U-1 and U-2, a total of seven battleships, six cruisers, eight destroyers, 28 torpedo boats, and four submarines ready for combat.

On 11 January 1918, U-1 was declared obsolete alongside her sister ship, but was retained as a training boat at the Austro-Hungarian submarine base located on Brioni Island.

[4] In mid-1918, U-1 was considered a potential candidate for service as a minesweeper, as the diving chamber present on the submarine could allow divers to sever the anchoring cables of sea mines.

Thus Emperor Karl I's government in Vienna asked the State of Slovenes, Croats and Serbs for help maintaining the fleet stationed at Pola and keeping order among the navy.

[29] Emperor Karl I, still attempting to save the Empire from collapse, agreed to the transfer, provided that the other "nations" which made up Austria-Hungary would be able to claim their fair share of the value of the fleet at a later time.

[30] All sailors not of Slovene, Croatian, Bosnian, or Serbian background were thus placed on leave for the time being, while the officers were given the choice of joining the new navy or retiring.

Furthermore, the State of Slovenes, Croats and Serbs had also not yet publicly rejected Emperor Karl I, keeping alive the possibility of reforming the Empire into a triple monarchy.

The transfer to the State of Slovenes, Croats and Serbs took place on the morning of 31 October, with Rear Admiral (German: Konteradmiral) Miklós Horthy meeting representatives from the South Slav nationalities aboard his flagship, Viribus Unitis.

[33] The State of Slovenes, Croats and Serbs lacked the men and officers to hold the fleet as most sailors who were not South Slavs had already gone home.

At a conference at Corfu, the Allied Powers agreed the transfer of Austria-Hungary's Navy to the State of Slovenes, Croats and Serbs was invalid, despite sympathy from the United Kingdom.

[34] Faced with the prospect of being given an ultimatum to surrender the former Austro-Hungarian warships, the National Council agreed to hand over the ships beginning on 10 November 1918.