Aragon's King Alfonso III is known for setting up the Militare Ordine di Collare di Sant?Agata dei Patern?. This entity was set up for defending Minorca as a knightly association. This took place on 23rd January 1289. The Fortress of Saint Agatha was the abode of the knights and this led to their naming.

Each of the knights was given a land adequate for maintaining a horse and armour. The knightly plots of land were active in the seventeenth century and some of them were even seen in the 19th century. King James I of Aragon founded the Royal House to which Alfonso III belonged. The dynasty ruled over places like Catalonia, the Balearic Islands, Aragon, and Sicily until 1410. It was during that time King Martin I passed away without any issue and the power passed away from the hands of the dynasty.

The Royal House's headship continued through the Princes of Cassano and the House of Ayerbe until 1834. But when the last Prince of Cassano passed away childless the situation took a turn. Ferdinand II, the contemporary Bourbon King called the several wings of the dynasty to find a head for the family. The conclave reached the conclusion that HRH don Mario should be awarded the headship of the family. The Order of the Collar of Saint Agatha was re-founded as a dynastic Order in the year 1855.

The Bourbon King of the Two Sicilies recognised the validity of the Order of the Collar of Saint Agatha in 1851. For a century starting from the year 1860 the Order continued to be a Family Order. As a matter of fact during this time the membership was kept within the limits of close associates and relatives. In 1961 the Order was reconstituted. An international organisation came into the forefront. Subsequently new Statutes were issued and a number of revisions also took place.

In Italy the Royal House's position was acknowledged in Rivista Araldica 1922 and 1913. It was also recognized in Libro d'Oro 1922 as sovereign foreign family staying in Italy. There are three Italian Courts that have declared that the Military Order of the Collar as well as the Royal House are legitimate entities. In December 1952 the United Court in Bari declared the legal status of the Royal House. In Pistoia in 1964 the Court of Appeal tried to evaluate if the Order was contrary to the Italian law nr 178. It found the order to be legitimate.

The same issue came to the focus when the Ordinary Tribunal of Ragusa tried to probe the matter and status of the Order in 2003. Professor Jacob Sundberg who is hailed as one of the stalwarts of public law in Europe has also compared the Royal House's head with a Head of State.