Casimir IV Jagiellon in the context of "John I Albert"

The Jagiellonian (US: /ˌjɑːɡjəˈloʊniən/ YAH-gyə-LOH-nee-ən) or Jagellonian dynasty (US: /ˌjɑːɡəˈ-/ YAH-gə-; Lithuanian: Jogailaičių dinastija; Polish: dynastia jagiellońska), otherwise the Jagiellon dynasty (Polish: dynastia Jagiellonów), the House of Jagiellon (Polish: Dom Jagiellonów), or simply the Jagiellons (Lithuanian: Jogailaičiai; Polish: Jagiellonowie; Latin: Iagellonidae), was the name assumed by a cadet branch of the Lithuanian ducal dynasty of Gediminids upon reception by Jogaila, the Grand Duke of Lithuania, of baptism as Ladislaus in 1386, which paved the way to his ensuing marriage to the Queen Regnant Hedwig of Poland, resulting in his ascension to the Crown of the Kingdom of Poland as Ladislaus II Jagiełło (initially ruling jure uxoris jointly with Jadwiga until her death), and the effective promotion of his branch to a royal dynasty. The Jagiellons were polyglots and per historical evidence Casimir IV Jagiellon and his son Saint Casimir possibly were the last Jagiellons who spoke in their patrilineal ancestors' Lithuanian language; however, even the last patrilineal Jagiellonian monarch Sigismund II Augustus maintained two separate and equally lavish Lithuanian-speaking and Polish-speaking royal courts in Lithuania's capital Vilnius. The Jagiellons reigned in several European countries between the 14th and 16th centuries. Members of the dynasty were Kings of Poland (1386–1572), Grand Dukes of Lithuania (1377–1392 and 1440–1572), Kings of Hungary (1440–1444 and 1490–1526), and Kings of Bohemia and imperial electors (1471–1526).

The personal union between the Kingdom of Poland and the Grand Duchy of Lithuania (converted in 1569 with the Treaty of Lublin into the Polish–Lithuanian Commonwealth) is the reason for the common appellation "Poland–Lithuania" in discussions about the area from the Late Middle Ages onward. One Jagiellon, Ladislaus III of Poland, briefly ruled both Poland and Hungary (1440–1444), and two others ruled both Bohemia and Hungary (1490–1526) and then continued in the distaff line as a branch of the House of Habsburg.

The Thirteen Years' War (Polish: wojna trzynastoletnia; German: Dreizehnjähriger Krieg), also called the War of the Cities, was a conflict fought in 1454–1466 between the Crown of the Kingdom of Poland and the Teutonic Order.

After the enormous defeat suffered by the German Order at the hand of Poland-Lithuania in 1410 and the ensuing political, military and economic problems, the state was rife with internal conflict between the ruling Order and the native Prussian warlords, who shared concerns with assimilated Prussian and German townsfolk. Eventually this tension led to an uprising by the Prussian Confederation representing the local Prussian nobility and cities, who sought the protection of the Polish King Casimir IV Jagiellon. This essentially amounted to a switching of sides which the German Order immediately took as a mortal threat, and a war broke out between Poland and the Teutons.

Casimir Jagiellon (/ˈkæzɪmɪər/; Latin: Casimirus; Lithuanian: Kazimieras; Polish: Kazimierz; 3 October 1458 – 4 March 1484) was a prince of the Kingdom of Poland and of the Grand Duchy of Lithuania. The second son of King Casimir IV Jagiellon, he was tutored by Johannes Longinus, a Polish chronicler, diplomat, and priest. After his elder brother Vladislaus was elected as King of Bohemia in 1471, Casimir became the heir apparent. At the age of 13, Casimir participated in the failed military campaign to install him as King of Hungary. He became known for his piety, devotion to God, and generosity towards the sick and poor. He became ill (most likely with tuberculosis) and died at the age of 25. He was buried in Vilnius Cathedral. His canonization was initiated by his brother King Sigismund I the Old in 1514 and the tradition holds that he was canonized in 1521. Saint Casimir the Prince is the patron saint of Poland, Lithuania and Lithuanian youth and the only saint with this name.

Veneration of Casimir saw a resurgence in the 17th century when his feast day was confirmed by the pope in 1602 and the dedicated Chapel of Saint Casimir was completed in 1636. Casimir became a patron saint of Lithuania and Lithuanian youth. In Vilnius, his feast day is marked annually with Kaziuko mugė (a trade fair) held on the Sunday nearest to 4 March, the anniversary of his death. There are more than 50 churches named after Casimir in Lithuania and Poland, including Church of St. Casimir, Vilnius and St. Kazimierz Church, Warsaw, and more than 50 churches in Lithuanian and Polish diaspora communities in America. Women's congregation Sisters of Saint Casimir was established in 1908 and remains active in the United States.

The rule of the Jagiellonian dynasty in Poland between 1386 and 1572 spans the Late Middle Ages and the Early Modern Period in European history. The Lithuanian Grand Duke Jogaila (Władysław II Jagiełło) founded the dynasty; his marriage to Queen Jadwiga of Poland in 1386 strengthened an ongoing Polish–Lithuanian union. The partnership brought vast territories controlled by the Grand Duchy of Lithuania into Poland's sphere of influence and proved beneficial for both the Polish and Lithuanian people, who coexisted and cooperated in one of the largest political entities in Europe for the next four centuries.

In the Baltic Sea region, Poland engaged in ongoing conflict with the Teutonic Knights. The struggles led to a major battle, the Battle of Grunwald of 1410, but there was also the milestone Peace of Thorn of 1466 under King Casimir IV Jagiellon; the treaty defined the basis of the future Duchy of Prussia. In the south, Poland confronted the Ottoman Empire and the Crimean Tatars, and in the east Poles helped Lithuania fight the Grand Duchy of Moscow. Poland's and Lithuania's territorial expansion included the far north region of Livonia.

The Peace of Thorn or Toruń of 1466, also known as the Second Peace of Thorn or Toruń (Polish: drugi pokój toruński; German: Zweiter Friede von Thorn), was a peace treaty signed in the Hanseatic city of Thorn (Toruń) on 19 October 1466 between the Polish king Casimir IV Jagiellon and the Teutonic Knights, which ended the Thirteen Years' War, the longest of the Polish–Teutonic Wars.

The treaty was signed in the Artus Court, and afterward a mass was held in the Gothic Franciscan Church of the Assumption of the Blessed Virgin Mary to celebrate the peace treaty.