The danger posed to the two realms by the Teutonic Order lent a particular value to the Lithuano-Polish alliance. For a couple of decades the Order had pursued aggressive policies towards Lithuania whilst seeking a reconciliation with Poland, counting on the rivalry of the two neighbours. They united in the face of the threat posed to Poland by the alliance of the Order with Sigismund of Luxemburg who, as margrave of Brandenburg, stood between Great Poland and Danzig, and in the face of border conflicts on the river Notec. Lithuania could not accept the loss of Zemaitija forced upon Vytautas in 1398, and in 1409 it supported a Zemaitijan uprising against the Order.
Similarly, the Order decided upon a recourse to military methods. The Knights concluded an alliance with Sigismund of Luxemburg, now king of Hungary, his brother King Vaclav of Bohemia and the dukes of Western Pomerania. The ‘Great War’ of 1409—11 between Poland—Lithuania and the Order culminated in the battle of Grunwald (Tannenberg) in 1410. This, one of the greatest battles of the late Middle Ages (it involved some 60,000 men), ended with the defeat of the Order’s forces. The grand master, Ulrich von Jungingen, and many Teutonic dignitaries were slain. But the peace settlement of 1411 satisfied only the war aims of Lithuania, which regained Zemaitija.
None the less the military and economic power of the Order was considerably weakened, to the advantage of the enhanced prestige of the allied peoples. From circles favouring church reform and from Jan Hus, who regarded the Order as an anachronism, there came letters of congratulations to King Wladyslaw. A fresh act of union was drawn up in 1413 at Horodlo on the river Bug, where forty-three Polish clans adopted a corresponding number of Lithuanian noble families, endowing them with Polish crests. King Wladyslaw and Grand Duke Vytautas granted the Lithuanian nobles fiscal and legal privileges on the Polish model.
The Polish delegation to the Council of Constance began to play an active role there in 1415. Representatives from the recently converted Zemaitijans attended the Council as witnesses of the success of Wladyslaw and Vytautas in carrying out their Catholic mission, while the metropolitan of Kiev attended as a representative from the grand duchy. The Poles were to come to the defence of Jan Hus. They presented the treatise of Paulus Vladimiri (of the University of Cracow) concerning papal and imperial power over the infidel. This canonist opposed conversion by the sword and defended the rights of pagans to own land, thereby provoking a bitter polemic with the supporters of the Teutonic Order. The Polish argument was backed by scholars from the University of Paris. Yet another war with the Order broke out in, but in 1422 the Knights were compelled to abandon Zemaitija once and for all. The Order’s expansion in the Baltic region was contained, and Prussia and Livonia were separated from each other.
The Polish—Lithuanian federation now became the great power of central and eastern Europe. Following the outbreak of the Hussite War in Bohemia in 1420, Czech groups which supported ideas of national monarchy moderated by social policies endorsed the candidature of Wladyslaw II Jagiello as king of Bohemia. The king declined this proposal in the face of complications abroad and opposition at home from Polish nobles who were unwilling to support Hussite sympathies among the gentry. With the king’s knowledge Vytautas received a similar offer, but he nominated Wladyslaw’s nephew, Prince Sigismund, son of Koributas, in his stead. Help for the Hussite rebellion was thwarted by the bishops and nobles headed by Zbigniew of Olesnica, bishop of Cracow, already a prominent political figure. The king had to recall Prince Sigismund and, in 1424, he was compelled to issue an edict against the Hussites and their allies. Nevertheless, in the following year Sigismund was to assume the role of king-elect and lent his support to the uprising of radical Hussite-Taborites in Silesia.
Following the birth of Crown Prince Wlladysllaw, whose mother was Wlladysllaw’s fourth and last wife, Sophia, from the Lithuanian ducal family of Alseniskis, the king entered discussions with the nobility concerning the succession to the Polish crown. The king’s son had an assured inheritance in Lithuania, for Vytautas had no heir, but in Poland the king had to secure his son’s right to succeed him by granting privileges, in particular to the nobility, which circumscribed royal power. Of several charters which defined the legal status of the nobility over the course of centuries, the most important was the Privilege of Brest (in Kujawy) of 1425, known by its opening words as Neminem captivabimus nisi iure victum. This guaranteed that noble property would not be confiscated, nor would nobles be imprisoned, without due legal process.
After the death in 1430 of Vytautas, who had spread his influence on the Lithuano-Rus'ian border deep into Muscovy and had even considered accepting the royal crown which Sigismund of Luxemburg offered him, King Wladyslaw placed the last of his living brothers, Svitrigaila, on the grand-ducal throne. He professed full independence from the kingdom of Poland, and allied himself with the Emperor Sigismund and the Teutonic Order which carried out a destructive raid on northern Poland. In 1432 Polish leaders managed to foment opposition to him in Lithuania which led to the fall of Svitrigaila’s regime and the seizure of the grand-ducal throne by Vytautas’s brother, Sigismund, son of Kestutis, who resumed the policy of union with Poland. A lasting achievement of Svitrigaila’s brief reign was the granting of equal rights to Catholic and Orthodox boyars — until that time only Catholics had enjoyed such privileges.