John Carr 
Knights Hospitaller [PDF ebook] 
A Military History of the Knights of St John

The Knights of St John evolved during the Crusades from a monastic order providing hostels for Christian pilgrims visiting the Holy Land. The need to provide armed escorts to the pilgrims began their transformation into a Military Order. Their fervour and discipline made them an elite component of most Crusader armies and Hospitaller Knights (as they were also known) took part in most of the major engagements, including Hattin, Acre and Arsuf. After the Muslims had reconquered the Crusader Kingdoms, the Order continued to fight from a new base, first in Rhodes and then in Malta. Taking to the sea, the Hospitallers became one of the major naval powers in the Mediterranean, defending Christian shipping from the Barbary Pirates (and increasingly turning to piracy themselves as funding from their estates in Europe dried up). They provided a crucial bulwark against Islamic expansion in the Mediterranean, obstinately resisting a massive siege of Malta by the Ottoman Turks in 1565. The Order remained a significant power in the Mediterranean until their defeat by Napoleon in 1798.