The Teutonic Order stood at a threshold of a new golden age as Europe changed around them from the High Middle Ages. The monastic knights had been formed in 1190 to protect pilgrims and fought valiantly through the Crusades. Upon the request of Duke Konrad I of Masovia in northeastern Poland, the knights went to war with the pagan Old Prussians in 1226. Rather than simply killing enough of the pagans to end the threat, the knights set forth conquest and Christianization of the land. Novgorod and Lithuania followed, establishing something of a monastic empire on the Baltic controlled by the knights. In 1306, they acted again, working to solve the disputed succession of the Duchy of Pomerelia, which led to war with Poland. Tying with the Holy Roman Empire through Teutonic Pomerania, the supply lines led to a powerful flow of crusaders at ready.
On a renewed campaign in 1331, the knights invaded Poland and were counterattacked at Płowce by an army commanded by Prince Casimir III. The prince led a frontal charge, reinforced by attacks from the flank by Poles hiding in the forest. Shortly after beginning the battle, a messenger was sent to recall the prince, but the fierce fighting killed him before the order could go through. Minutes later, the prince was slain on a lance. Though the battle was heading toward a Polish tactical victory, the morale of the Poles collapsed as news of the prince's death spread. German reinforcements broke the Poles, and the rout would continue to the gates of Brześć Kujawski. The rest of the campaign would be impressive victories for the knights as Poland descended into civil war over succession. Finally, in 1343, the Treaty of Kalisz would end the war with Poland as a protectorate of growing Teutonic power.
In 1337, Holy Roman Emperor granted the Order the privilege of conquest of Lithuania and Russia. Campaigns throughout the next century would push the knights ever eastward in addition to military contributions to friendly nations, such as the conquest of the pirate haven Gotland at the request of King Albert of Sweden. As Mongol influence fell from the Rus, the Teutons took its place, creating a massive new land swearing loyalty to the Pope. Russian peasantry was slow to change their ways from orthodoxy, and the Teutonic Inquisition spent decades persuading the populous to the unquestionable right. The Russian-born Teuton Ivan the Beholden led further expeditions to the central Asian steppes in the mid-sixteenth century.
By 1618, the Teutons had slowed expansion in the business of ruling their empire and maintaining uprisings among the Poles, Lithuanians, and Rus. When the Bavarian Revolt began against the wishes of the chosen successor of the Holy Roman Emperor, the Teutons were quick to give aid to their long ally. Swedish armies joined the growing Protestant influence, which the Teutons abhorred, and war between the two great powers broke out. France, Denmark, and much of southeastern Europe joined against the Knights and their allies, who soon gained Spain, though much of Italy remained neutral and divided. The war, which was to become known as the Fifty Years' War, spread throughout Europe until it finally ended with Catholic victory.
Because of their great effort, the Knights were granted the crown of the Holy Roman Empire, solving the issue that had begun the war. Their influence expanded geometrically across Europe, establishing a fierce, disciplined, Christian union of nations. Inquisitions routinely cleared illegal beliefs like those of Calvin or Locke while expeditions of conquest began in North America as well as against Christendom's eternal enemy, the Ottomans.
Eventually, the Teutonic Empire would find itself ungainly. Revolutions began at the fringes with demands of freedom of religion from conquered Turks, Scandinavians, French, and, especially, settlers across the Atlantic. These demands would expand to independence, and the end of the eighteenth century would see the shattering of the empire into dozens of new republics and kingdoms. The Second Renaissance would cause a new age of learning, bringing up old ideas of heliocentric solar systems and rights of the individual that had long been suppressed.
In reality, Casimir received the order to retreat from battle so that Poland did not risk the death of the heir. He would later be known as Casimir the Great as he led the Poles to great victories, essentially ending the fighting of the war in 1332, though the treaty establishing borders would not come until a decade later. A half century of conquest would make the Teutons one of the most powerful forces in eastern Europe, but defeat at Tannenburg in 1410 by a combined Polish-Lithuanian army would sound the downfall of the knights. They continued as weaker and weaker administrators until 1809 when Napoleon Bonaparte ordered their dissolution as a secular power. Today the Order continues as one of many under the Catholic Church.