The Knights of the Virgin Mary – or Teutonic Knights of the Hospital of St. Mary, the Virgin – was a religious-military Order that that was founded at the siege of Acre in 1190 during the Third Crusade. German in origin, its original purpose was to care for and protect German pilgrims to the Holy Land. Its knights took vows of poverty, chastity and obedience. They wore a white mantle with a black cross. The Order gained the approval of the German Emperor 1191 and its conversion to a military order was approved by the pope in 1199.
About 1225 a Mazovian duke of Poland employed them to assist in his invasion of the then heathen Prussians. By the 1280’s, helped by their absorption of a purely military order, the Brethren of the Sword, they had conquered most of the area of what is now northern Poland and the Baltic states in a sort of perpetual on-growing crusade-cum-invasion that, in the end, was to bring about their downfall as the political implications of their power began to out-weigh the religious. By the 1380s they were, effectively, the most powerful Catholic state in Northern and Central Europe, ruling from a series of great castles.
Their control of already converted states caused alienation and they were soundly defeated in battle by Christian Polish-Lithuanian forces in 1410. They went into decline, were secularised in Prussia in 1525, and were finally abolished as any sort of force by Napoleon in 1809.
The Order was revived in 1834, and transformed to a purely mendicant one in 1929.