the Knights of Malta - when a small Christian guerilla army defeated an empire

The Knights Hospitaller/ knights of saint John or Knights of Malta were up against the Ottomans they faced an immense army of over 40,000 men and at least an equal number of slaves. The Ottomans first attempted to take Malta in 1551 but failed. In 1565, Suleiman the Magnificent, the Ottoman Sultan, made a second attempt to take Malta. The Knights, who numbered around 500 together with approximately 6,000 footsoldiers, withstood the siege and repelled the invaders.
This victory became one of the most celebrated events of sixteenth-century Europe, to the point that Voltaire said: "Nothing is better known than the siege of Malta." It undoubtedly contributed to the eventual erosion of the European perception of Ottoman invincibility, although the Mediterranean continued to be contested between Christian coalitions and the Muslim Turks for many years.