The Teutonic Order was founded in the 12th century as a monastic order to care for German-speakers in the Holy Land. Eventually, it transformed into a military order to fight the Muslims.

They were later requested to defend Christians by Andrew II, King of Hungary in the early 13^th century. They fought the nomadic Cumans in Transylvania and built many castles and towns to defend the mountain passes of the Carpathians. They also invited German settlers to settle there to boost the defense of those towns and fortifications.

The Teutonic Order demanded autonomy from the Kingdom of Hungary, which worried Andrew II. They were also rapidly gaining power in the area. He kicked them out in 1225, just fourteen years after he had invited them. The Teutonic Order then looked for work elsewhere.

Poland was in a poor condition at the time. Civil wars and dynastic disputes were very common in the early monarchy. As a result, King Boleslaw III divided the kingdom among his sons to prevent any more succession disputes. Instead, his successors fought with each other even more, greatly weakening Poland as a whole and making it more susceptible to invasion. To the north, the Baltic Prussians resisted missionary efforts and raided the Polish Duchy of Mazovia in response.

Due to the fragmentation, Konrad II, duke of Mazovia lacked an army strong enough to stop the raids. In 1226, he called on the Teutonic Order to help him stabilize his northern border and gave them land in Chelmno as a base for their operations in the area. Hermann von Salza, the head of the Order, felt that Prussia would be a good training ground for later involvement in the Holy Land and agreed The Teutonic Order managed to stop the Prussian raids and even conquered Prussian territory.

As a result, Frederick II, Holy Roman Emperor, passed the Golden Bull of Rimini in 1226, which allowed them to take as much of Prussia as they wanted and also let them keep Chelmno. The Pope also issued charters declaring it to be a sovereign monastic state, similar to the Knights Hospitaller in Malta. This amounted to protection of the Order by the HRE and the papacy, legitimizing their conquests and preventing other states from challenging them for the time being. The Order managed to take control of the area after 50 years of fighting, in which many of the Prussian natives were brutally killed. Prussian revolts against the Order led to even more deaths on the Prussian side, but the Order's control of the area grew stronger and stronger.

Much like in Hungary, German settlers were invited to colonize the area. In the Order's holdings many castles were built to suppress Prussian revolts, while new German-settled towns and any German-dominated countryside became bastions of stability. Prussia now became solidly under the Order's control, allowing it to make offensives elsewhere.

Their next target was the Baltic lands in what is now Latvia and Estonia. The pagan Lithuanians had defeated a fellow crusading order, the Livonian Brothers of the Sword who were established in the area at Saule in 1236, forcing them out of their conquests. The remnants of the Brothers of the Sword joined the Teutonic Order as an autonomous branch called the Livonian Order the following year. Much like the wars with the Baltic Prussians, the Teutonic Order spent a long time conquering the Baltic pagans, invited German settlers, and brutally suppressed rebellions.

They also launched offensives against the fragmented Rus' states, which ended in 1242 after the Battle of the Ice. Alexander Nevsky, Prince of Novgorod, held off the Order's armies on a frozen lake. When the Order's armies later retreated, their heavy armor caused the ice to crack, drowning many of the knights. This defeat set the border between Catholicism and Orthodoxy in the Baltic.

With the fall of the last Crusader territory in 1291, the Order could focus more on its campaigns in the Baltic. The Teutonic Order began to fight Lithuania. The Order conquered a some territory on the border, but Lithuania remained pagan until 1387- 150 years after Saule. Its sovereignty, however, was never damaged. Lithuania became a strong enemy of the Order as well.

The Teutonic Order later became more secular in its conquests, especially with its wars against Catholic Poland. In 1308, a succession dispute over Pomerelia between Poland and Brandenburg erupted into war. Poland called on the Order to force Brandenburg out, but the Order just took the land for itself, including its biggest city, Danzig. Polish-Teuton wars would continue for two centuries, and Poland became another strong enemy of the Order.

Poland and Lithuania grew closer together, united by a common hatred of the Teutons. A personal union of Poland and Lithuania formed in 1385 under the Lithuanian Jagellonian dynasty. Lithuania was converted to Catholicism soon after, as Catholic Poland would not have pagan monarchs. This removed any justification for the Order's wars with Lithuania and galvanized the anti-Order alliance.

This alliance would be tested in the Polish-Lithuanian-Teutonic war of 1409-11. At the Battle of Grunwald, the Teutonic Order was soundly defeated. Although they ended the war with only minimal territorial concessions, they never regained their former power. The Polish-Lithuanian alliance became dominant in the area, conquering more and more land from the Order after continuous victories. The Livonian branch took advantage of the Order's weakness to become completely independent in 1435, causing the Teutonic Order to lose all of its lands north of the Memel.

Further territory was lost in the Thirteen Years' War of 1454-66. In the Second Peace of Thorn, Danzig was regained by Poland, as well as Western Prussia, also known as Royal Prussia. Eastern Prussia was retained by the Order as a Polish fief. The Order then became a Polish vassal. Subsequent attempts by the Order to revise the treaty failed.

The last of these attempts was the 1519-21 war. Four years later, Albert of Brandenburg, the Grand Master of the Teutonic Order, converted to Lutheranism and secularized the Order's holdings as the Duchy of Prussia. Poland supported this as Albert willingly submitted to Polish overlordship in exchange for recognition. The new Duchy of Prussia also voided the Imperial and Papal charters supporting the Teutonic Order.

As the Order was a Catholic military organization, it was kicked out from the area. It moved to Austria, a Catholic country, but did not undertake any military operations. It became a purely religious order in 1929. It was suppressed by the Nazis, despite German propaganda of the time extolling the virtues of the Teutonic Order. It outlasted the war and still exists today, providing religious guidance and caring for the sick in German-speaking communities in Germany, Italy, Austria, Slovenia, the Czech Republic, and Slovakia.

They also have their own website: http://www.imperialteutonicorder.com/

Sources:

Rosamond McKitterick's Atlas of the Medieval World and http://www.imperialteutonicorder.com/