In 1226, Polish Prince Conrad of Mazovia (Mazovia is a place in Northern Poland) asked the Teutonic Knights from Transylvania to come to Mazovia. He wanted them to fight the Prussian tribes on his borders. They fought for more than 100 years. Then they created a new state. After some time, this state controlled most of today's Estonia, Latvia, and Lithuania, and parts of northern Poland. In 1466, the Knights were under the King of Poland and Lithuania. In 1525, the leader of the Knights became a Protestant. He made part of the Knights' land into the Duchy of Prussia, which was then part of the Kingdom of Poland. 