Balto-Slavic past of Kaliningrad should be included in the rich history of this strategically important region of Europe. Thus the early interest in establishing an international rule of law in Poland was caused by a forgery committed by the German Armed Brethren, who obtained a temporary fief of Chełmno from Konrad I of Mazovia (1187–1247) in 1228 in the Act of Kruszwica. The temporary fief of Chełmno was obtained by the Teutonic Knights for the time needed to convert the Balto-Slavic Prussians to Christianity.
The forgery by the Teutonic Knights consisted of changing the text of the Act of Kruszwica into a permanent grant of the fief of Chełmno. This forgery caused the successive kings of Poland to look for an international legal procedure to amend the forgery and avoid an armed conflict over Prussia in which the Teutonic Knights committed genocide of the Balto-Slavic Prussians and violated the Christian principle that the license to convert is not a license to kill.
Eventually, the military triumphs of the union of Poland and Lithuania over the Teutonic Knight were soon paralleled by successes in diplomacy. After their defeat by the Polish king, the armed monks of the Teutonic Order accused Poland of killing German missionaries and allying itself with pagans. Theses accusations were to be investigated at the Council of Constance, (1414–1418), one of the great diplomatic conferences of the Middle Ages.
Paweł Włodkowicz, (Paulus Vladimiri) of Brudzewo, Polish ambassador at the Council of Constance, served also as President of the University of Kraków. He was a Professor of Law. In 1415 at the Council, Włodkowic proposed the first seventeen basic theses of international law founded on justice and toleration. His proposal was based on the natural law and the premise that the license to convert is not a license to kill or expropriate and that only voluntary conversion is valid. He defined the principle of national self-determination, the international society, its functions, organs, and laws. He began to formulate these laws for use by an international tribunal, which he proposed. He justified only purely defensive wars. W³odkowicz advocated international mediation and arbitration and an international tribunal for the peaceful solution of conflicts among nations. He argued that the Teutonic Order of armed monks lost its missionary character by committing mass murders and pillage. Therefore, in reality, the German Order constituted a “Prussian heresy.” On the other hand the Christianization of Lithuania by Poland represented the greatest medieval missionary deed.
The Council of Constance accepted the arguments of the Polish Ambassador. The Establishing of due process under the law followed in Poland some 250 years earlier than in England. The due legal process guaranteed the inviolability of citizen's person (who was not caught in the act of committing a crime). It was formulated in Poland for the first time in Europe, in the acts of 1422-1433. This due process was the basis of the legal system in Poland when absolutism reigned in the rest of Europe.
German aggression on the Baltic coast caused the formation of two unions of states. In the south, Poland and Lithuania united at Krewo in 1385. In the north all of Scandinavia united twelve years later at Kalmar in 1397.
In the Union Act of Krewo, Jogaiła or Jagiełło, the Grand Duke of Lithuania, committed himself to convert Lithuania to Latin Christianity and to unite with Poland all Lithuanian and Ruthenian lands as well as to recover Polish territories lost to the Germans, in exchange for marriage to Polish Queen Jadwiga of Anjou and his coronation as a Catholic King of Poland. Before his coronation in 1386, Władys³aw Jagiełło, Ladislas Jogaiła (1350–1434), confirmed the Act of Koszyce (1374). During the reign of King Władysław Jagiełło (1386–1434), Polish missionaries converted Lithuania to Roman Catholicism. In 1387 the Act of Wilno followed. In it the King Władysław Jagiełło bestowed hereditary ownership of land and freedom from taxation by the local princes of the newly converted Catholic Lithuanian nobles, the boyars. Moldavia became a fief of Poland. In 1388 in the Act of Piotrków, King Władysław Jagiełło increased the civil rights of nobility and clergy, further limiting the royal power in Poland.
The tyrannical rule of the German monastic orders resulted in the founding of the “Salamander Society” or “Reptile Association” in 1397. It was a forerunner of the Prussian Union, which was organized for the overthrow of the rule of the Teutonic Order and for the unification in freedom of Prussia with Poland.
In 1401 in the Union Act of Wilno and Radom the Lithuanian knighthood received the same civil rights as were enjoyed by Polish knights. Poland guaranteed the safety of Lithuania against the aggression by the Armed Brethren of the TeutonicOrder in 1409.Victory and a new union act of Poland and Lithuania in 1410 in the largest medieval battle of Lannenberg-Grunwald
The Great War against the Teutonic Order by Poland and Lithuania lasted two years (1409–1411). On June 30, 1410 the decisive victory in the battle of Tannenberg-Grunwald was won by the Polish and Lithuanian forces assisted by Czech Hussites and auxiliaries from Smoleñsk. The chief of the Teutonic Order Eric von Jungingen was killed on the battlefield. Second victory on October 10, 1410 at Koronowo led to peace negotiations. The terms of the Peace Treaty of Toruñ included the return of the province of Dobrzyñ to Poland and of the province of Żmudź to Lithuania and payment to Poland of money in the amount of 6,000,000 groszes by the Teutonic Order. The victory transformed the Polish-Lithuanian union into a great power and put an end to the expansionist plans of the Teutonic Order and the Luxemburgers; it put an end to German aggression in the Baltic area.
In the Union Act of Horodło (1413) was concluded by the King Władysław Jagiełło, following the victory over German Monastic State of the Teutonic Knights in 1410. It was a personal union of Poland and Lithuania, which were to remain two separate states. The King established the territorial office of wojewoda (vo-ye-vo-da) or provincial governor, and initiated a new administrative and defensive organizational model, which was followed in central and eastern Europe. (The ancient term wojewoda, meaning one who leads warriors, is still in use as an administrative title.) Polish families extended the use and privileges of their coats of arms to the Lithuanian and Ruthenian clans.
The frontiers of Western civilization were shifted considerably to the northeast and Polish social and political institutions penetrated Lithuania, Belrus, and Kievian Ruthenia. New towns were founded and were granted a wide measure of self-government. Considerable prosperity was achieved by Polish towns in the 14th and 15th centuries. Those that were members in the Hanseatic League shared in the profitable Baltic and Levantine trade.
European balance of power changed as a result of the union of Poland and Lithuania which became the largest territory in Europe.