The allied victory at the Battle of Grunwald in 1410, followed by the Peace of Thorn, secured the Polish and Lithuanian borders and marked the emergence of the Polish–Lithuanian alliance as a significant force in Europe.

The Muscovites' Pyrrhic victory over the Golden Horde, in the long term, signified, however, the beginning of a slow climb to power by the Grand Duchy of Moscow, which became within a century the most serious rival and threat to the integrity, well-being and survival of Lithuania.

In the north-west, Lithuania faced constant armed incursions from the Teutonic Knights—founded after 1226 to fight and convert the pagan Baltic tribes of Prussians, Yotvingians and Lithuanians.

[6] Jogaila's Russian mother Uliana of Tver urged him to marry Sofia, daughter of Prince Dmitri of Moscow, who required him first to convert to Orthodoxy.

[15] The Polish nobles saw the offer as an opportunity for increasing their privileges[16] and avoiding Austrian influence, brought by Jadwiga's previous fiancé William, Duke of Austria.

Though the ethnic Lithuanian nobility were the main converts to Catholicism—both paganism and the Orthodox rite remained strong among the peasants—the king's conversion and its political implications created lasting repercussions for the history of both Lithuania and Poland.

[26] Jagiello's baptism failed to end the crusade of the Teutonic Knights, who claimed his conversion was a sham, perhaps even heresy, and renewed their incursions on the pretext that pagans remained in Lithuania.

[9][27] From then on, however, the Order found it harder to sustain the cause of a crusade and faced the growing threat to its existence posed by the Kingdom of Poland and a genuinely Christian Lithuania alliance.

[9] The decision may not have improved Władysław's relations with the Order, but it served to introduce closer ties between Lithuania and Poland, enabling the Polish church to freely assist its Lithuanian counterpart.

On 4 September 1390, the joint forces of Vytautas and Grand Master Konrad von Wallenrode of the Teutonic Order, laid siege to Vilnius, which was held by Władysław's regent Skirgaila with combined Polish, Lithuanian and Ruthenian troops.

[22][32] The protracted period of war between the Lithuanians and the Teutonic Knights was ended on 12 October 1398 by the Treaty of Salynas, named after the islet in the Neman River where it was signed.

[22] Shortly afterwards, Vytautas was crowned as a king by local nobles; but the following year his forces and those of his ally, Khan Tokhtamysh of the White Horde, were crushed by the Timurids at the Battle of the Vorskla River, ending his imperial ambitions in the east and obliging him to submit to Władysław's protection once more.

In 1402, Władysław answered the rumblings against his rule by marrying Anna of Cilli, a granddaughter of Casimir III of Poland, a political match that re-legitimized his reign.

[33][34] Since no heir had yet been produced by either monarch, the implications of the union were unforeseeable, but it forged bonds between the Polish and Lithuanian nobility and a permanent defensive alliance between the two states, strengthening Lithuania's hand for a new war against the Teutonic Order in which Poland officially took no part.

[27] On 31 January 1402, he presented himself in Marienburg, where he won the backing of the Knights with concessions similar to those made by Jogaila and Vytautas during earlier leadership contests in the Grand Duchy.

[33] Both sides had practical reasons for signing the treaty at that point: the Order needed time to fortify its newly acquired lands, the Poles and Lithuanians to deal with territorial challenges in the east and in Silesia.

[36] In December 1408, Władysław and Vytautas held strategic talks in Navahrudak Castle, where they decided to foment a Samogitian uprising against Teutonic rule to draw German forces away from Pomerelia.

[37] The uprising, which began in May 1409, at first provoked little reaction from the Knights, who had not yet consolidated their rule in Samogitia by building castles; but by June their diplomats were busy lobbying Władysław's court at Oborniki, warning his nobles against Polish involvement in a war between Lithuania and the Order.

[38] Władysław, however, bypassed his nobles and informed the new Grand Master Ulrich von Jungingen that if the Knights acted to suppress Samogitia, Poland would intervene.

[42] When the war resumed in June 1410, Władysław advanced into the Teutonic heartland at the head of an army of about 20,000 mounted nobles, 15,000 armed commoners, and 2,000 professional cavalry mainly hired from Bohemia.

[43] The road to the Teutonic capital Marienburg now lay open, the city undefended; but for reasons the sources do not explain, Władysław hesitated to pursue his advantage.

[46][47] The apparent half-heartedness of the ensuing siege, called off by Władysław on 19 September, has been variously ascribed to the impregnability of the fortifications,[46] high Lithuanian casualties, to Władysław's unwillingness to risk further casualties, or to his desire to keep the Order weakened but undefeated so as to not upset the balance of power between Poland (which would most likely acquire most of the Order possessions if it was totally defeated) and Lithuania; but a lack of sources precludes a definitive explanation.

[50] In an effort to outflank his critics, Władysław promoted the leader of the opposing faction, bishop Mikołaj Trąba, to the archbishopric of Gniezno in autumn 1411 and replaced him in Kraków with Vytautas supporter Wojciech Jastrzębiec.

Vytautas sent a delegation in 1415, including the metropolitan of Kiev and Samogitian witnesses; they arrived at Constance at the end of that year to express their preference for being "baptised with water and not with blood".

[52] The Polish envoys, among them Mikołaj Trąba, Zawisza Czarny, and Paweł Włodkowic, lobbied for an end to the forced conversion of heathens and to the Order's aggression against Lithuania and Poland.

[53] As a result of the Polish–Lithuanian diplomacy, the council, though scandalised by Włodkowic's questioning of the legitimacy of the monastic state, denied the Order's request for a further crusade and instead entrusted the conversion of the Samogitians to Poland–Lithuania.

[54] The diplomatic context at Constance included the revolt of the Bohemian Hussites, who looked upon Poland as an ally in their wars against Sigismund, the emperor elect and new king of Bohemia.

In 1421, the Bohemian Diet declared Sigismund deposed and formally offered the crown to Władysław on condition that he accept the religious principles of the Four Articles of Prague, which he was not prepared to do.

[34] In 1432, a pro-Polish party in Lithuania elected Vytautas's brother Žygimantas as grand duke,[19] leading to an armed struggle over the Lithuanian succession which stuttered on for years after Władysław's death.

[61][19] He finally died in Grodek in 1434, leaving Poland to his elder son, Władysław III, and Lithuania to his younger, Casimir, both still minors at the time.