Sigismund III Vasa (Polish: Zygmunt III Waza, Lithuanian: ygimantas Vaza; 20 June 1566 30 April 1632

N.S.) was King of Poland and Grand Duke of Lithuania from 1587 to 1632 and, as Sigismund, King of Sweden and Grand Duke of Finland from 1592 to 1599. He was the first Polish sovereign from the House of Vasa. A religious zealot, he imposed Roman Catholic doctrine across the vast realm, and his crusades against neighbouring states marked Poland's largest territorial expansion. As an enlightened despot, he presided over an era of prosperity and achievement, further distinguished by the transfer of the country's capital from Krakw to Warsaw.

Sigismund was the son of King John III of Sweden and his first wife, Catherine Jagiellon, daughter of King Sigismund I of Poland. Elected monarch of the PolishLithuanian Commonwealth in 1587, he sought to unify Poland and Sweden under one Catholic kingdom, and when he succeeded his deceased father in 1592 the PolishSwedish union was created. Opposition in Protestant Sweden caused a war against Sigismund headed by Sigismund's uncle Charles IX, who deposed him in 1599.

Sigismund attempted to hold absolute power in all his dominions and frequently undermined parliament. He suppressed internal opposition, strengthened Catholic influence and granted privileges to the Jesuits, whom he employed as advisors and spies during the Counter-Reformation. He actively interfered in the affairs of neighbouring countries; his invasion of Russia during the Time of Troubles resulted in brief control over Moscow and seizure of Smolensk. Sigismund's army also defeated the Ottoman forces in southeastern Europe, which hastened the downfall of Sultan Osman II. However, the PolishSwedish conflict had a less favourable outcome. After a series of skirmishes ending in a truce, King Gustavus Adolphus of Sweden launched a campaign against the Commonwealth and annexed parts of Polish Livonia.

Sigismund remains a controversial figure in Poland. One of the country's most recognisable monarchs, his long reign coincided with the Polish Golden Age, the apex in the prestige, power and economic influence of the PolishLithuanian Commonwealth. On the other hand, it was also during his rule that the seeds of decline surfaced. Considerable contributions to the arts and architecture as well as military victories were tarnished by intrigues and religious persecutions. He was commemorated in Warsaw by Sigismund's Column, one of the city's chief landmarks and the first secular monument in the form of a column in modern history. It was commissioned after Sigismund's death by his son and successor, Wadysaw IV.

The Polish–Lithuanian Commonwealth, formally known as the Kingdom of Poland and the Grand Duchy of Lithuania, and, after 1791, as the Commonwealth of Poland, was a country and bi-federation of Poland and Lithuania ruled by a common monarch in real union, who was both King of Poland and Grand Duke of Lithuania. It was one of the largest and most populous countries of 16th to 17th-century Europe. At its largest territorial extent, in the early 17th century, the Commonwealth covered almost 1,000,000 square kilometres (400,000 sq mi) and as of 1618 sustained a multi-ethnic population of almost 12 million. Polish and Latin were the two co-official languages.

The Commonwealth was established by the Union of Lublin in July 1569, but the Crown of the Kingdom of Poland and the Grand Duchy of Lithuania had been in a de facto personal union since 1386 with the marriage of the Polish queen Jadwiga (Hedwig) and Lithuania's Grand Duke Jogaila, who was crowned King jure uxoris Władysław II Jagiełło of Poland. The First Partition in 1772 and the Second Partition in 1793 greatly reduced the state's size and the Commonwealth was partitioned out of existence with the Third Partition in 1795.

The Union possessed many features unique among contemporary states. Its political system was characterized by strict checks upon monarchical power. These checks were enacted by a legislature (sejm) controlled by the nobility (szlachta). This idiosyncratic system was a precursor to modern concepts of democracy, as of 1791 constitutional monarchy, and federation. Although the two component states of the Commonwealth were formally equal, Poland was the dominant partner in the union.The Polish–Lithuanian Commonwealth was marked by high levels of ethnic diversity and by relative religious tolerance, guaranteed by the Warsaw Confederation Act 1573; however, the degree of religious freedom varied over time. The Constitution of 1791 acknowledged Catholicism as the "dominant religion", unlike the Warsaw Confederation, but freedom of religion was still granted with it.After several decades of prosperity, it entered a period of protracted political, military, and economic decline. Its growing weakness led to its partitioning among its neighbors (Austria, Prussia, and Russia) during the late 18th century. Shortly before its demise, the Commonwealth adopted a massive reform effort and enacted the 3 May Constitution, which was the first codified constitution in modern European history and the second in modern world history after the United States Constitution.