Bear in mind that for several centuries the Sovereign Order was in an almost constant "running battle" with the forces of Islam.  Beginning in 1440, several major assaults were made on Rhodes by the Egyptian Mamelukes.  All were carried out with much devastation and indescribable atrocities to the Christian population.  Ultimately, in each of these invasions, the Knights inflicted defeat, heavy losses and humiliation on the attackers.  However, on three occassions - in 1480 and again in 1522 on Rhodes, and in 1565 on Malta, the Muslims concentrated all their numbers and power against the homeland of the Knights in order to eradicate them.  The purpose was clear.  If the Knights of Saint John were destroyed, Islam could then invade and subjugate all of Christian Europe.  This goal became the obsession of the Turks, and resulted in the Three Great Sieges which became the high points of the Order's glorious history.