The early modern era of Polish history follows the Late Middle Ages. Historians use the term early modern to refer to the period beginning in approximately 1500 AD and lasting until around the Napoleonic Wars in 1800 AD.

The Nihil novi act adopted by the Polish diet in 1505 transferred legislative power from the king to the diet. This event marked the beginning of the period known as the "Nobles' Democracy" () or "Nobles' Commonwealth" (). The state was ruled by the "free and equal" Polish nobility or szlachta, albeit in intense, and at times destabilizing, competition with the Jagiellon and then elective kings.

The Union of Lublin of 1569 constituted the Polish–Lithuanian Commonwealth, a more closely merged continuation of the already existing personal union of the Crown of Poland and the Grand Duchy of Lithuania. The beginning of the Commonwealth coincided with the period of Poland's greatest territorial expansion, power, civilizational advancement and prosperity. The Polish–Lithuanian state had become an influential player in Europe and a vital cultural entity, spreading Western culture eastward.

Following the Reformation gains accompanied by religious toleration, the Catholic Church embarked on an ideological counter-offensive and Counter-Reformation claimed many converts from Protestant circles. The disagreements over and the difficulties with the assimilation of the eastern Ruthenian populations of the Commonwealth had become clearly discernible; an attempt to settle the issue was made in the religious Union of Brest. On the military front, a series of Cossack uprisings took place.

The Commonwealth, assertive militarily under King Stephen Báthory, suffered from dynastic distractions during the reigns of the Vasa kings Sigismund III and Władysław IV. It had also become a playground of internal conflicts, in which the kings, powerful magnates and factions of nobility were the main actors. The Commonwealth fought wars with Russia, Sweden and the Ottoman Empire.

The situation, however, soon radically deteriorated. From 1648 the Cossack Khmelnytsky Uprising engulfed the south and east, and was soon followed by a Swedish invasion, which raged through core Polish lands. Warfare with the Cossacks and Russia left Ukraine divided, with the eastern part, lost by the Commonwealth, becoming the Tsardom's dependency. John III Sobieski, fighting protracted wars with the Ottoman Empire, revived the Commonwealth's military might once more, in the process helping decisively in 1683 to deliver Vienna from a Turkish onslaught.

The Commonwealth, subjected to almost constant warfare until 1720, suffered devastating population losses, massive damage to its economy and social structure. The government became ineffective because of large scale internal conflicts (e.g. Lubomirski's Rokosz against John II Casimir and other confederations), corrupted legislative processes (liberum veto) and manipulation by foreign interests. The "ruling" nobility class fell under control of a handful of powerful families with established territorial domains. The reigns of two kings of the Saxon Wettin dynasty, Augustus II and Augustus III, brought the Commonwealth further disintegration.

The Polish-Lithuanian state was dominated by the Russian Empire from the time of Peter the Great. This foreign control reached its climax under Catherine the Great, and involved at that time also the Kingdom of Prussia and the Austrian Habsburg monarchy. During the later part of the 18th century the Commonwealth recovered economically, developed culturally and attempted fundamental internal reforms. The reform activity provoked hostile reaction and eventually military response on the part of the neighboring powers. The royal election of 1764 resulted in the reign of Stanisław August Poniatowski.

The Bar Confederation of 1768 was a szlachta rebellion directed against Russia and the Polish king. It was brought under control and followed in 1772 by the First Partition of the Commonwealth, a permanent encroachment on the outer Commonwealth provinces by Russia, Prussia and Austria.

The Great, or Four-Year Sejm was convened by Stanisław August in 1788. The Sejm's landmark achievement was the passing of the Constitution of May 3, 1791, considered the first in modern Europe. The constitutional reform generated strong opposition from conservative circles in the Commonwealth's upper nobility and from Catherine II.

The nobility's Targowica Confederation appealed to Empress Catherine for help and in May 1792 the Russian army entered the territory of the Commonwealth. The defensive war fought by the forces of the Commonwealth ended when the King, convinced of the futility of resistance, capitulated by joining the Targowica Confederation. Russia and Prussia in 1793 arranged for and executed the Second Partition of the Commonwealth, which left the country with critically reduced territory, practically incapable of independent existence.

Reformers and patriots were soon preparing for a national insurrection. Tadeusz Kościuszko, chosen as its leader, on March 24, 1794, in Cracow (Kraków) declared a national uprising. Kościuszko emancipated and enrolled in his army many peasants, but the hard-fought insurrection ended in suppression by the forces of Russia and Prussia. The third and final partition of the Commonwealth was undertaken again by all three partitioning powers, and in 1795 the Polish–Lithuanian Commonwealth ceased to exist.

Early elective monarchy

Non-hereditary royal succession

The death of Sigismund II Augustus in 1572 ended the nearly two centuries of the rule of the Jagiellon dynasty in Poland. It was followed by a three-year interregnum period, during which the Polish nobility (szlachta) was searching for ways to continue the governance process and elect a new monarch. Lower szlachta was now included in the selection process and adjustments were made to the constitutional system. The power of the monarch was further circumscribed in favor of the expanding noble class, which sought to ensure its future domination.

Each king had to sign the so-called Henrician Articles (named after Henry of Valois, the first post-Jagiellon king), which were the basis of the political system of Poland, and the pacta conventa, which were various further personal obligations of the chosen king. From that point, the king was effectively a partner with the nobility, a top member of the diet (sejm), and was constantly supervised by a group of upper-rank nobles, senators from sejm's upper chamber.

The disappearance of the ruling dynasty and its replacement with a non-hereditary elective monarchy made the constitutional system much more unstable. With each election the noble electors wanted more power for themselves and less for the monarch, although there were practical limits to how much the kings could be constrained. A semi-permanent power struggle resulted, to which the magnates and lesser szlachta added their own constant manipulations and bickering and authority eroded from the government's center. Eventually foreign states had taken advantage of the vacuum and replaced the nobility of the Commonwealth as the real arbiter of royal elections and of overall power in Poland-Lithuania.

In its periodic opportunities to fill the throne, the szlachta exhibited a preference for foreign candidates who would not found another strong dynasty. This policy produced monarchs who were either ineffective or in constant debilitating conflict with the nobility. The kings of alien origin were initially unfamiliar with the internal dynamics of the Commonwealth, had remained distracted by the politics of their native countries, and often inclined to subordinate the interests of the Commonwealth to those of their own country and ruling house.

Henry of Valois (1573–1574)

In April 1573, Sigismund's sister Anna, the sole heir to the crown, convinced the Sejm to elect the French prince Henry of Valois as king. Her marriage with Henry was to further legitimize Henry's rule, but less than a year after his coronation, Henry fled Poland to succeed his brother Charles IX as King of France.

Stephen Báthory (1576–1586)

The able and militarily as well as domestically assertive Transylvanian Stephen Báthory (1576–1586) counts among the few more highly regarded elective kings.

During the Livonian War (1558–1582), fought between Ivan the Terrible of Russia and Poland-Lithuania, Pskov was besieged by Polish forces. The city was not captured, but Báthory, with his Chancellor Jan Zamoyski, led the Polish army in a decisive campaign and forced Russia to return territories previously taken, gaining Livonia and Polotsk. In 1582, the war ended with the Truce of Jam Zapolski.

The Commonwealth forces retrieved most of the lost provinces. At the end of Báthory's reign, Poland ruled two main Baltic Sea ports: Danzig (Gdańsk), controlling the Vistula River trade and Riga, controlling the Daugava River trade. Both cities were among the largest in the country.

War of the Polish Succession

Stephen Báthory planned a Christian alliance against the Islamic Ottomans. He proposed an anti-Ottoman alliance with Russia, which he considered necessary for his anti-Ottoman crusade. Russia however was heading for its Time of Troubles and he could not find a partner there. When Báthory died, there was a year-long interregnum. Emperor Mathias' brother, Archduke Maximilian III, tried to claim the Polish throne, but was defeated at Byczyna during the War of the Polish Succession (1587–1588). Sigismund III Vasa became the Commonwealth's next king, the first of the three rulers from the Swedish House of Vasa.

House of Vasa

Sigismund III Vasa (1587–1632)

Sigismund III Vasa was King of Poland 1587–1632 and King of Sweden 1592–1599. He was the son of John III Vasa of Sweden and Catherine, the daughter of Sigismund I the Old of Poland. He annoyed the Polish nobles by deliberately dressing in Spanish and other Western European styles (including French hosiery).Peter K. Gessner, The reign of the Vasa dynasty (1587-1668), State University of New York at Buffalo An ardent Catholic, Sigismund III was determined to win the Swedish crown and bring Sweden back to Catholicism. Subsequently, Sigismund III involved Poland in unnecessary and unpopular wars with Sweden during which the diet refused him money and soldiers and Sweden seized Livonia and Prussia.

The first few years of Sigismund's reign (until 1598) saw Poland and Sweden united in a personal union that made the Baltic Sea an internal lake. However, a rebellion in Sweden started the chain of events that would involve the Commonwealth in more than a century of warfare with Sweden.

The Catholic Church embarked on an ideological counter-offensive and Counter-Reformation claimed many converts from Protestant circles. The Union of Brest split the Eastern Christians of the Commonwealth. In order to further Catholicism, the Uniate Church (acknowledging papal supremacy but following Eastern ritual and Slavonic liturgy) was created at the Synod of Brest in 1596. The Uniates drew many followers away from the Orthodox Church in the Commonwealth's eastern territories.

Sigismund's attempts to introduce absolutism, then becoming prevalent in the rest of Europe, and his goal of reacquiring the throne of Sweden for himself, resulted in a rebellion of the szlachta (gentry). In 1607, the Polish nobility threatened to suspend the agreements with their elected king but did not attempt his overthrow.

For ten years between 1619 and 1629, the Commonwealth was at its greatest geographical extent in history. In 1619, the Russo-Polish Truce of Deulino came into effect, whereby Russia conceded Commonwealth control over Smolensk and several other border territories. In 1629, the Swedish-Polish Truce of Altmark took place; the Commonwealth ceded to Sweden most of Livonia, which the Swedes had invaded in 1626.