The Order of the Knights of Saint John, also known as Order of Saint John, Order of Hospitallers, Knights Hospitaller, and the Hospitallers, were among the most famous of the Roman Catholic military orders during the Middle Ages.
The Hospitallers probably arose as a group of individuals associated with an Amalfitan hospital in the Muristan district of Jerusalem, which was dedicated to St John the Baptist and founded around 1023 by Blessed Gerard Thom to provide care for poor, sick or injured pilgrims to the Holy Land. (Some scholars, however, consider that the Amalfitan order and Amalfitan hospital were different from Gerard's order and its hospital.) After the Latin Christian conquest of Jerusalem in 1099 during the First Crusade, the organisation became a religious and military order under its own Papal charter, and it was charged with the care and defence of the Holy Land. Following the conquest of the Holy Land by Islamic forces, the Order operated from Rhodes, over which it was sovereign, and later from Malta where it administered a vassal state under the Spanish viceroy of Sicily.
The Order was weakened in the Reformation, when rich commanderies of the Order in northern Germany and the Netherlands became Protestant (and, largely separated from the Roman Catholic main stem, remain so to this day), and the Order was disestablished in England, Denmark, and elsewhere in northern Europe. The Roman Catholic order was further damaged by Napoleon's capture of Malta in 1798 and became dispersed throughout Europe. It regained strength during the early 19th century as it redirected itself toward humanitarian and religious causes. In 1834, the order, by this time known as the Sovereign Military Order of Malta (SMOM), acquired new headquarters in Rome where it has remained since.
Five contemporary, state-recognised chivalric orders which claim modern inheritance of the Hospitaller tradition all assert that "The Sovereign Military Order of Malta is the original order" and that four non-Roman Catholic orders stem from the same root: Protestant orders exist in Germany, the Netherlands and Sweden, and a non-denominational British revival is headquartered in the United Kingdom.