Sigismund II Augustus in the context of "Jagiellons"

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.

The Union of Lublin (Polish: Unia lubelska; Lithuanian: Liublino unija) was signed on 1 July 1569 in Lublin, Poland, and created a single state, the Polish–Lithuanian Commonwealth, one of the largest countries in Europe at the time. It replaced the personal union of the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania with a real union and an elective monarchy, as Sigismund II Augustus, the last of the Jagiellons, remained childless after three marriages. In addition, the autonomy of Royal Prussia was largely abandoned. The Duchy of Livonia, tied to Lithuania in real union since the Union of Grodno (1566), became a Polish–Lithuanian condominium.

The Commonwealth was ruled by a single elected monarch who carried out the duties of King of Poland and Grand Duke of Lithuania, and governed with a common Senate and parliament (the Sejm). The Union is seen by some as an evolutionary stage in the Polish–Lithuanian alliance and personal union, necessitated also by Lithuania's dangerous position in wars with Russia.

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.

Jan Sariusz Zamoyski (Latin: Ioannes Zamoyski de Zamoscie; 19 March 1542 – 3 June 1605) was a Polish nobleman, magnate, statesman and the 1st ordynat of Zamość. He served as the Royal Secretary from 1565, Deputy Chancellor from 1576, Grand Chancellor of the Crown from 1578, and Great Hetman of the Crown from 1581.

Zamoyski was the General Starost of the city of Kraków from 1580 to 1585, Starost of Bełz, Międzyrzecz, Krzeszów, Knyszyn and Tartu. An important advisor to Kings Sigismund II Augustus and Stephen Báthory, he was one of the major opponents of Bathory's successor, Sigismund III Vasa, and one of the most skilled diplomats, politicians and statesmen of his time, standing as a major figure in the politics of the Polish–Lithuanian Commonwealth throughout his life.

The Union of Grodno established a real union between the Grand Duchy of Lithuania and the Duchy of Livonia on 25 December 1566, during the Livonian War. Livonia had submitted itself to Sigismund II Augustus by the Treaty of Vilnius (1561). Livonia's administrative division was re-organized with its castellans becoming members of the Lithuanian senate. The union did not impact Livonian jurisdiction, which was to be carried out according to its traditional customs.

Sigismund I the Old (Polish: Zygmunt I Stary, Lithuanian: Žygimantas I Senasis; 1 January 1467 – 1 April 1548) was King of Poland and Grand Duke of Lithuania from 1506 until his death in 1548. Sigismund I was a member of the Jagiellonian dynasty, the son of Casimir IV and younger brother of Kings John I Albert and Alexander I Jagiellon. He was nicknamed "the Old" in later historiography to distinguish him from his son and successor, Sigismund II Augustus. Before ascending to the Polish and Lithuanian thrones, he was Duke of Głogów from 1499, Duke of Opava from 1501, and governor of Silesia from 1504 on behalf of his brother, King Vladislaus II of Bohemia and Hungary.

Sigismund was born in the town of Kozienice in 1467 as the fifth son of Casimir IV and his wife Elizabeth of Austria. He was one of thirteen children and was not expected to assume the throne after his father. Sigismund's eldest brother and rightful heir Vladislaus II instead became the King of Bohemia, Hungary and Croatia as the successor to George of Poděbrady in Bohemia and then to Matthias Corvinus in Hungary, thus temporarily uniting these kingdoms. When Casimir died, the Polish-Lithuanian realm was divided between the remaining two older sons, with John Albert being crowned King of Poland, and Alexander as Grand Duke of Lithuania. Alexander inherited Poland following John Albert's sudden death in 1501. Hence, Sigismund's reign only began when he succeeded Alexander to both titles in 1506 at the age of 39.