The Sacred Military Constantinian Order of St. George is an international Catholic order of chivalry under the grand mastership of His Royal Highness Prince Pedro of Bourbon Two Sicilies, Duke of Castro.  The Order had already received numerous signs of Papal recognition and support  from the middle of the 16th century and, by the end of the 17th century, it had members throughout much of Italy, with a large community in Spain as well as smaller numbers of knights in Austria, Hungary, Poland, Croatia (then part of the Kingdom of Hungary), and even the Spanish-American colonies. Pope Clement XI, in the 1718 papal bull Militantis Ecclesiae, confirmed the Order as a religious-military order of the Catholic Church, and it was the beneficiary of many subsequent papal privileges. In 1910 Pope Pius X restored the office of Cardinal Protector of the Constantinian Order, the last incumbent of which died in 1927 (the position of Cardinal Protector no longer exists in canon law), while in 1915 Pope Benedict XV dedicated the chapel of the Order in the Basilica of Santa Croce al Flaminio, which remains in use by the Order today. Among the members of the Constantinian Order who contributed to the construction of the chapel was Monsignor Eugenio Pacelli, later elected Pope as Pius XII. 

The provable history of the Order dates from the mid-16th century.  In 1698, the grand mastership of the Order passed to the House of Farnese, in the person of Francesco Farnese, Duke of Parma and was invested in the Farnese family and its heirs by Pope Clement XI.  In 1731, the grand mastership passed to the Royal House of Bourbon as heirs of the Farnese, where it has remained ever since.