Battle of Grunwald
Names and sources: 

Names: Traditionally, the battle's location was thought to be in the territory of the monastic state of the Teutonic Order, on the plains between three villages: Grünfelde (Grunwald) to the west, Tannenberg (Stębark) to the northeast and Ludwigsdorf (Łodwigowo, Ludwikowice) to the south. However, research by Swedish historian Sven Ekdahl and archaeological excavations in 2014–2017 proved that the actual site was south of Grünfelde (Grunwald). Władysław II Jagiełło referred to the site in Latin as in loco conflictus nostri, quem cum Cruciferis de Prusia habuimus, dicto Grunenvelt. Later, Polish chroniclers interpreted the word Grunenvelt ("green field" in Low German) as Grünwald, meaning "green forest" in German. The Lithuanians followed suit and translated the name as Žalgiris. The name Žalgiris was first used by Maironis in 1891. The Germans named the battle after Tannenberg ("fir hill" or "pine hill" in German). Thus, there are three commonly used names for the battle: German: Schlacht bei Tannenberg, Polish: bitwa pod Grunwaldem, Lithuanian: Žalgirio mūšis. Its names in the languages of other involved peoples include Belarusian: Бітва пад Грунвальдам, Ukrainian: Грюнвальдська битва, Russian: Грюнвальдская битва, Czech: Bitva u Grunvaldu, Romanian: Bătălia de la Grünwald.

Sources: There are few contemporary, reliable sources about the battle, and most were produced by the Polish. The most important and trustworthy source is Cronica conflictus Wladislai regis Poloniae cum Cruciferis anno Christi 1410, which was written within a year of the battle by an eyewitness. Its authorship is uncertain, but several candidates have been proposed: Polish deputy chancellor Mikołaj Trąba and Władysław II Jagiełło's secretary Zbigniew Oleśnicki. While the original Cronica conflictus did not survive, a short summary from the 16th century has been preserved. Historiae Polonicae by Polish historian Jan Długosz (1415–1480). is a comprehensive and detailed account written several decades after the battle. The reliability of this source suffers not only from the long gap since the events, but also from  Długosz's alleged biases against Lithuanians. Banderia Prutenorum is a mid-15th-century manuscript with images and Latin descriptions of the Teutonic battle flags captured during the battle and displayed in Wawel and Vilnius Cathedrals. Other Polish sources include two letters written by Władysław II Jagiełło to his wife Anne of Cilli and Bishop of Poznań Wojciech Jastrzębiec and letters sent by Jastrzębiec to Poles in the Holy See. German sources include a concise account in the chronicle of Johann von Posilge. An anonymous letter, discovered in 1963 and written between 1411 and 1413, provided important details on Lithuanian maneuvers.

Historical background: 

Lithuanian Crusade and Polish–Lithuanian union: In 1230, the Teutonic Order, a crusading military order, moved to Chełmno Land (Kulmerland) and launched the Prussian Crusade against the pagan Prussian clans. With support from the pope and Holy Roman Emperor, the Teutons conquered and converted the Prussians by the 1280s and shifted their attention to the pagan Grand Duchy of Lithuania. For about 100 years, the order raided Lithuanian lands, particularly Samogitia, as it separated the order in Prussia from their branch in Livonia. While the border regions became an uninhabited wilderness, the order gained very little territory. The Lithuanians first gave up Samogitia during the Lithuanian Civil War (1381–84) in the Treaty of Dubysa.
In 1385, Grand Duke Jogaila of Lithuania agreed to marry Queen Jadwiga of Poland in the Union of Kreva. Jogaila converted to Christianity and was crowned King of Poland and became known as Władysław II Jagiełło, thus creating a personal union between the Kingdom of Poland and the Grand Duchy of Lithuania. The official Lithuanian conversion to Christianity removed the religious rationale for the order's activities in the area. Its grand master, Conrad Zöllner von Rothenstein, supported by Hungarian King Sigismund of Luxemburg, responded by publicly contesting the sincerity of Jogaila's conversion, bringing the charge to a papal court. The territorial disputes continued over Samogitia, which had been in Teutonic hands since the Peace of Raciąż in 1404. Poland also had territorial claims against the order in Dobrzyń Land and Gdańsk (Danzig), but the two states had been largely at peace since the Treaty of Kalisz (1343). The conflict was also motivated by trade considerations: the order controlled the lower reaches of the three largest rivers (the Neman, Vistula and Daugava) in Poland and Lithuania.

War, truce and preparations: In May 1409, an uprising in Teutonic-held Samogitia started. Lithuania supported it and the order threatened to invade. Poland announced its support for the Lithuanian cause and threatened to invade Prussia in return. As Prussian troops evacuated Samogitia, Teutonic Grand Master Ulrich von Jungingen declared war on the Kingdom of Poland and the Grand Duchy of Lithuania on 6 August 1409. The order hoped to defeat Poland and Lithuania separately, and began by invading Greater Poland and Kuyavia, catching the Poles by surprise. The order burned the castle at Dobrin (Dobrzyń nad Wisłą), captured Bobrowniki after a 14-day siege, conquered Bydgoszcz (Bromberg) and sacked several towns. The Poles organized counterattacks and recaptured Bydgoszcz. The Samogitians attacked Memel (Klaipėda). However, neither side was ready for a full-scale war.
Wenceslaus, King of the Romans, agreed to mediate the dispute. A truce was signed on 8 October 1409 and was set to expire on 24 June 1410. Both sides used this time to prepare for war, gathering troops and engaging in diplomatic maneuvering. Both sides sent letters and envoys accusing each other of various wrongdoings and threats to the Christendom. Wenceslaus, who received a gift of 60,000 florins from the order, declared that Samogitia rightfully belonged to the order and only Dobrzyń Land should be returned to Poland. The order also paid 300,000 ducats to Sigismund of Hungary, who had ambitions regarding the Principality of Moldavia, for mutual military assistance. Sigismund attempted to break the Polish–Lithuanian alliance by offering Vytautas a king's crown; Vytautas' acceptance would have violated the terms of the Ostrów Agreement and created Polish-Lithuanian discord. At the same time, Vytautas managed to obtain a truce from the Livonian Order.
By December 1409, Władysław II Jagiełło and Vytautas had agreed on a common strategy: their armies would unite into a single massive force and march together towards Marienburg (Malbork), capital of the Teutonic Order. The order, who took a defensive position, did not expect a joint attack and were preparing for a dual invasion—by the Poles along the Vistula River towards Danzig (Gdańsk) and the Lithuanians along the Neman River towards Ragnit (Neman). To counter this perceived threat, Ulrich von Jungingen concentrated his forces in Schwetz (Świecie), a central location from where troops could respond to an invasion from any direction rather quickly. Sizable garrisons were left in the eastern castles of Ragnit, Rhein (Ryn) near Lötzen (Giżycko) and Memel (Klaipėda). To keep their plans secret and mislead the order, Władysław II Jagiełło and Vytautas organized several raids into border territories, thus forcing the order to keep their troops in place.

Opposing forces: The precise number of soldiers involved has proven difficult to establish. None of the contemporary sources provided reliable troop counts. Jan Długosz provided the number of banners, the principal unit of each cavalry: 51 for the Teutons, 50 for the Poles and 40 for the Lithuanians. However, it is unclear how many men were under each banner. The structure and number of infantry units (pikemen, archers, crossbowmen) and artillery units is unknown. Estimates, often biased by political and nationalistic considerations, were produced by various historians.