The Militare Ordine di Collare di Sant'Agata dei Patern? was founded by King Alfonso III of Aragon on the 23rd January 1289 as a knightly association with the purpose of defending Minorca. The member knights were obliged to live in the Fortress of Saint Agatha, and so became known as "the knights from Saint Agatha". They were each allocated a plot of land sufficient to maintain them in arms with a horse ("cavalleria"). The cavallerias were still active in the year 1600 and some even survived into the 19th century. When the 7th Duke of Carcaci travelled to the Balearics in the mid 18th century the remaining descendants told him of this ancient chivalric institution known as "the knights from Saint Agatha".



The Royal House to which Alfonso III belonged was founded by King James I of Aragon, "James the Conqueror". They reigned over Aragon, Catalonia, Valencia, the Balearic Islands and Sicily until 1410 when King Martin I died without children and the dynasty lost power due to a coup d'?tat known as the "compromise" of Caspe in 1413. The headship of the Royal House continued through the House of Ayerbe and the Princes of Cassano until 1834, when the last Prince of Cassano died in Italy without children. The then reigning Bourbon King of The Two Sicilies, Ferdinand II, called upon the various and complex branches of the dynasty to summon a family conclave to decide who was now to be the head of the family. The decision of the conclave was that HRH don Mario was to succeed to the headship of the family and as Grand Master of the chivalric patrimony. In 1855 the Order of the Collar of Saint Agatha was re-established as a dynastic Order (family Order) on the foundation of the knights of Saint Agatha founded by King Alfonso III.

The legitimacy of the Order of the Collar of Saint Agatha was recognised early as 18th May 1851 by the Bourbon King of the Two Sicilies, whose officials were required to record its conferrals in the Registry Office of the Kingdom. On 30th March 1853 the Governor of the Province of Catania, in the name of the King, allowed only three exceptions to the Royal prohibition on the wearing of orders other than Royal Sicilian Orders, these being Papal Orders, the Order of Malta and the Military Order of the Collar. In 1859 the Royal Commission on Titles of Nobility examined the claims of the House of Patern? to confer titles and bestow Orders and it advised the King that such acts, titles and Orders were legitimate. In 1860 the reigning Head of State, H.M. King Francesco II, approved this decision by Royal Decree and ordered its execution throughout the Kingdom. The King also recognised Don Mario as hereditary Grand Master of the Order and made various provisions to ensure the succession to the Grand Magistracy. From 1860 to 1960 the Order remained almost a Family Order, largely restricted in its membership to relatives and close associates. The Order was reconstituted in 1961 with an international organisation, and new Statutes have been issued and revised on a number of occasions.

The position of the Royal House in Italy was recognised in the Rivista Araldica 1913 and 1922 and in the Libro d'Oro 1922 as being a foreign sovereign family residing in Italy. Three Italian Courts have tried and decided that the Royal House and the Military Order of the Collar are legitimate. The United Court in Bari had already on 13th December 1952 declared that this Royal House was legitimate including "the exercise of the Grand Magistry of the Orders of Chivalry conferred by their family". The Court of Appeal in Pistoia on 5th June 1964 specifically tried the question if this Order violates the Italian law nr 178 from 1951, and found that it does not. Again, the Ordinary Tribunal of Ragusa found on 28th January 2003 that the Head of this Royal House has "the quality of a subject of international law and of Grand Master of non-National Orders within the terms of the Law of the 3rd March 1951, no 178".

One of Europe's leading experts on public law, Professor Jacob Sundberg, has declared that the Head of the Royal House has rank equivalent to a Head of State. Among the members of the Order today are found Heads of State, Ambassadors, Bishops, Politicians etc.