Prince Aimone, Duke of Aosta

Designation of Aimone as King of Croatia on 18 May 1941. In front of him poglavnik Pavelić with the Croatian delegation

On 18 May 1941, in a ceremony at the Quirinal Palace, to which Ante Pavelić, the leader of the fascist Ustaše movement that had assumed power in Croatia in April 1941 after the invasion of Yugoslavia, led a delegation of Croats requesting that Italy's King Victor Emmanuel III name a member of the House of Savoy as King of Croatia. The Independent State of Croatia was a fascist puppet state that was partly under Italian and German control, covering most of present-day states of Croatia and Bosnia and Herzegovina, but its leaders tried to assert their legitimacy by instating a monarchy that would resemble the medieval Croatian state.

Aimone was then officially named King by his cousin Victor Emmanuel III.[21] On assuming the Crown of Zvonimir he took the regnal name Tomislav II in memory of Tomislav, the first Croatian king. Originally on learning that he had been named King of Croatia he told close colleagues that he thought his nomination was a bad joke by his cousin King Victor Emmanuel III, though he accepted the crown out of a sense of duty.[22] The Italian Foreign Minister and Benito Mussolini's son in law, and Count Ciano's informants said of Aimone "The Duke doesn't give a damn about Croatia and wants only money, money and more money."[23] Ciano's diary noted a conversation between Aimone and himself, where Aimone was "proud of having been chosen King of Croatia, but has no exact idea of what he is supposed to do and is vaguely uneasy about it".[24]

He was due to be crowned in Duvno (Tomislavgrad), in modern-day Bosnia and Herzegovina, but he refused to go to there due to the "Dalmatian question" which arose due to Italy taking some of Dalmatia's coastal territory. Aimone felt that Dalmatia "was a land that could never be Italianized" and was an obstacle to Italian-Croatian reconciliation.[25] Other reasons why he never went to Croatia were because of an ongoing insurgency, and that his safety could not be guaranteed.[23] Because of this he exercised what little power he had from Italy and Hungary,[26] however he never held any real authority throughout his reign as the Ustaše government had deprived the monarchy of most powers and reduced the status of the king to that of a figurehead.[22] In spite of this he did have some symbolic powers such as the ability to grant noble titles. Count Gyula István Cseszneky de Milvány et Csesznek was the counselor to the King for Croatian affairs. Prince Aimone also established a Croatian office in Rome where he received confidential reports, official documents, and military, political and economic information from Croatia.[27]

After the fall of the Fascist regime in Italy, Aimone abdicated as king of Croatia on 31 July 1943 on the orders of Victor Emmanuel III.[26][28][29][30]

Prince Aimone succeeded to the title Duke of Aosta on 3 March 1942, following the death of his elder brother Prince Amedeo, 3rd Duke of Aosta, in a British Prisoner of War camp in Kenya.

In the Autumn of 1942, Aimone contacted Allied forces via his courier, the consul general Alessandro Marieni, about the possibility of a peace settlement between Italy and allied forces.[31] Secret talks would continue into 1943, motivated in part by the aim of preserving the royal dynasty of Savoy.[31]

In the latter months of World War II, he became the commander of the Italian Naval Base of Taranto but he was dismissed from his post for his criticism of the judges that had found General Mario Roatta guilty of war crimes.[32] During his naval career he reached the rank of Squadron Admiral.

•  Independent State of Croatia: Sovereign Knight Commander of the Military Order of the Iron Trefoil, 1st Class
•  Independent State of Croatia: Sovereign Knight Grand Cross of the Order of the Crown of King Zvonimir, Special Class
•  Independent State of Croatia: Sovereign Recipient of the Medal of the Crown of King Zvonimir
• Italy: Knight Grand Collar of the Supreme Order of the Most Holy Annunciation[36][37]
• Italy: Knight Grand Cordon of the Order of Saints Maurice and Lazarus
• Italy: Knight Grand Cordon of the Order of the Crown of Italy
• Italy: Knight Grand Cordon of the Military Order of Savoy
• Italy: Knight of the Civil Decoration of Savoy
• Italy: Recipient of the Silver Medal of Military Valor
• Italy: Recipient of the Bronze Medal of Military Valor
• Italy: Recipient of the Medal of Military Valor
• Italy: Recipient of the Medal of Honour for Long-time Maritime Navigation
  • Sovereign Military Order of Malta: Bailiff Knight Grand Cross of Justice of the Sovereign Military Order of Malta, 1st Class

• Spain: Knight Grand Cross of the Order of Charles III[38]
• Greece: Knight Grand Cross of the Order of the Redeemer[37]
• Iran: Knight Grand Cordon of the Order of Pahlavi[39]
• Romania: Knight Grand Cross with Collar of the Order of Carol I[37]
• United Kingdom: Recipient of the Victory Medal