The history of the order for the next century presents little of interest. In 1388 two of the knights repaired to England by order of the grand master, to make commercial arrangements with that country, which had been rendered necessary by the changes introduced into the trade of Europe by the creation of the Hanseatic League. A second commercial treaty between the King of England and the order was made in 1409.
The order had now reached the summit of its greatness. Besides large possessions in Germany, Italy, and other countries, its sovereignty extended from the Oder to the Gulf of Finland. This country was both wealthy and populous. Prussia is said to have contained fifty-five large fortified cities, forty-eight fortresses, and nineteen thousand and eight towns and villages. The population of the larger cities must have been considerable, for we are told that in 1352 the plague carried off thirteen thousand persons in Dantzic, four thousand in Thorn, six thousand at Elbing, and eight thousand at Koenigsberg. One authority reckons the population of Prussia at this time at two million one hundred and forty thousand eight hundred. The greater part of these were German immigrants, since the original inhabitants had either perished in the war or retired to Lithuania.
Historians who were either members of the order or favorably disposed toward it, are loud in their praise of the wisdom and generosity of its government; while others accuse its members and heads of pride, tyranny, luxury, and cruel exactions.
In 1410 the Teutonic order received a most crushing defeat at Tannenberg from the King of Poland, assisted by bodies of Russians, Lithuanians, and Tartars. The grand master, Ulric de Jungingen, was slain, with several hundred knights and many thousand soldiers.
There is said to have been a chapel built at Gruenwald, in which an inscription declared that sixty thousand Poles and forty thousand of the army of the knights were left dead upon the field of battle. The banner of the order, its treasury, and a multitude of prisoners fell into the hands of the enemy, who shortly afterward marched against Marienberg and closely besieged it. Several of the feudatories of the knights sent in their submission to the King of Poland, who began at once to dismember the dominions of the order and to assign portions to his followers. But this proved to be premature. The knights found in Henry de Planau a valiant leader, who defended the city with such courage and obstinacy that, after fifty-seven days' siege, the enemy retired, after serious loss from sorties and sickness. A series of battles followed, and finally a treaty of peace was signed, by which the order gave up some portion of its territory to Poland.