Jadwiga of Poland in the context of "Polish-Lithuanian Commonwealth"

The Crown of the Kingdom of Poland (Polish: Korona Królestwa Polskiego; Latin: Corona Regni Poloniae) was a political and legal concept formed in the 14th century in the Kingdom of Poland, assuming unity, indivisibility and continuity of the state. Under this idea, the state was no longer seen as the patrimonial property of the monarch or dynasty, but became a common good of the political community of the kingdom. This notion allowed the state to maintain stability even during periods of interregnum and paved the way for a unique political system in Poland, characterized by a noble-based parliament and the free election of the monarch. Additionally, the concept of the Crown extended beyond existing borders, asserting that previously lost territories still rightfully belonged to it. The term Crown of the Kingdom of Poland also referred to all the lands under the rule of the Polish king. This meaning became especially significant after the union with the Grand Duchy of Lithuania, when it began to be commonly used to denote the Polish part of the joint Polish-Lithuanian Commonwealth.

The idea of the Crown in Central Europe first appeared in Bohemia and Hungary, from where the model was taken by kings Ladislaus the Short and Casimir III the Great to strengthen their power. During the reign of Louis the Great in Poland, who spent most of his time in Hungary, as well as during the interregnum following his death and the regency during the minority of his daughter Jadwiga, the idea was adopted by the lords of the kingdom to emphasize their own role as co-responsible for the state.

The Polish–Lithuanian Commonwealth, also referred to as Poland–Lithuania or the First Polish Republic (Polish: I Rzeczpospolita), was a federative real union between the Kingdom of Poland and the Grand Duchy of Lithuania, existing from 1569 to 1795. This state was among the largest, most populated countries of 16th- to 18th-century Europe. At its peak in the early 17th century, the Commonwealth spanned approximately 1,000,000 km (390,000 sq mi) and supported a multi-ethnic population of around 12 million as of 1618. The official languages of the Commonwealth were Polish and Latin, with Catholicism as the state religion.

The Union of Lublin established the Commonwealth as a single entity on 1 July 1569. The two nations had previously been in a personal union since the Krewo Agreement of 1385 (Polish–Lithuanian union) and the subsequent marriage of Queen Jadwiga of Poland to Grand Duke Jogaila of Lithuania, who was crowned as Władysław II Jagiełło, jure uxoris King of Poland. Their descendant, Sigismund II Augustus, enforced the merger to strengthen frontiers of his dominion and maintain unity. As he remained childless, his death in 1572 marked the end of the Jagiellonian dynasty. It introduced an elective monarchy, whereupon members of domestic noble families or external dynasties were elected to the throne for life.

In a strict sense, the Union of Krewo or Act of Krėva (also spelled Union of Krevo, Act of Kreva; Polish: unia w Krewie; Lithuanian: Krėvos sutartis) comprised a set of prenuptial promises made at Kreva Castle on 14 August 1385 by Jogaila, Grand Duke of Lithuania, in regard to his prospective marriage to the underage reigning Queen Jadwiga of Poland.

Though very limited in scope, the "Union of Krewo", in historiography, often refers not only to the particular document but to events of 1385–1386 as a whole. After the 1385 negotiations, Jogaila converted to Christianity, married Jadwiga, and was crowned King of Poland in 1386.

Jogaila (Lithuanian: [jɔˈɡâːɪɫɐ] ; c. 1352/1362 – 1 June 1434), later Władysław II Jagiełło (Polish: [vwaˈdɨswaf jaˈɡʲɛwwɔ] ), was Grand Duke of Lithuania beginning in 1377 and starting in 1386, becoming King of Poland as well. As Grand Duke, he ruled Lithuania from 1377 to 1381 and from 1382 to 1401, at which time he became the Supreme Duke of Lithuania in exchange for naming his cousin Vytautas as the new Grand Duke. Władysław II initially served as King of Poland alongside his wife Jadwiga until her death in 1399, and then the sole ruler until his own death in 1434.

Raised a Lithuanian polytheist, he converted to Catholicism in 1386 and baptized as Ladislaus (Polish: Władysław) in Kraków, married the young Queen Jadwiga, and was crowned King of Poland as Władysław II Jagiełło. In 1387, he converted Lithuania to Catholicism. His reign in Poland started in 1399, upon the death of Queen Jadwiga, lasted a further thirty-five years, and laid the foundation for the centuries-long Polish–Lithuanian union. He was a member of the Jagiellonian dynasty in Poland that bears his name and was previously also known as the Gediminid dynasty in the Grand Duchy of Lithuania. The dynasty ruled both states until 1572, and became one of the most influential dynasties in late medieval and early modern Europe.

Poland was ruled at various times either by dukes and princes (10th to 14th centuries) or by kings (11th to 18th centuries). During the latter period, a tradition of free election of monarchs made it a uniquely electable position in Europe (16th to 18th centuries).

The first Polish ruler whose existence is not debatable was Duke Mieszko I, who adopted Christianity under the authority of Rome in the year 966. He was succeeded by his son, Bolesław I the Brave, who greatly expanded the boundaries of the Polish state and ruled as the first king in 1025. The following centuries gave rise to the mighty Piast dynasty, consisting of both kings such as Mieszko II Lambert, Przemysł II or Władysław I the Elbow-high and dukes like Bolesław III Wrymouth. The dynasty's rule over Poland ceased with the death of Casimir III the Great in 1370. In the same year, the Capetian House of Anjou became the ruling house with Louis I as king of both Poland and Hungary. His daughter, Jadwiga, later married Jogaila, the pagan Grand Duke of Lithuania, who in 1386 was baptized and crowned as Władysław II Jagiełło, thus creating the Jagiellonian dynasty and a personal union between Poland and Lithuania.