Grand Duchy of Lithuania
Polish–Lithuanian Commonwealth

The wars with the Teutonic Order, the loss of land to Moscow, and the continued pressure threatened the survival of the state of Lithuania, so it was forced to ally more closely with Poland, forming a real union with the Kingdom of Poland in the Union of Lublin of 1569. The union was formally called the Kingdom of Poland and the Grand Duchy of Lithuania, however now commonly known as the Polish-Lithuanian Commonwealth. During the period of the Union, many of the territories formerly controlled by the Grand Duchy of Lithuania were transferred to the Crown of the Polish Kingdom, while the gradual process of Polonization slowly drew Lithuania itself under Polish domination.

Following the death of Grand Duke Sigismund II Augustus in 1572, a joint Polish–Lithuanian monarch was to be elected as in the Union of Lublin it was agreed that the title "Grand Duke of Lithuania" will be received by a jointly elected monarch in the Election sejm on his accession to the throne, thus losing its former institutional significance, however the Union of Lublin guaranteed that the institution and the title "Grand Duke of Lithuania" will be preserved.

In 1573, Henry Valua was elected as the first joint Polish–Lithuanian monarch, however his rule was short and he never personally visited the Grand Duchy of Lithuania, despite being announced as the Grand Duke of Lithuania.

The double election of 1575 was held in the presence of a small number of Lithuanian lords, who additionally supported the Habsburg candidate Emperor Maximilian II, however, the race for the crown was won by Stephen Báthory, crowned on May 1, 1576. The Lithuanian lords, at a convention in Grodno (on 8-20 April 1576), protested this choice, threatening to break the union and giving themselves the right to choose a separate ruler. However, the king managed to rally the Lithuanian delegation by promising to preserve their rights and freedoms. On May 29, 1580, in Vilnius Cathedral, King and Grand Duke Stephen Báthory received from the hand of the bishop of Samogitia Merkelis Giedraitis a blessed sword and hat, given by Pope Gregory XIII through the envoy Paweł Uchański. This was a recognition by the Pope of the ruler's successes in the struggle against the infidels. In Lithuania, this ceremony was treated as the celebration of the elevation of the Grand Duke of Lithuania, during which Lithuania's sovereignty was manifested. Báthory's reign was marked with successful Livonian campaign against tsar Ivan the Terrible 's military forces, which resulted in the reintegration of Polotsk to Lithuania and the restoration of control of the Duchy of Livonia.

The rule of Lithuania by the Gediminid–Jagiellonian family representatives resumed through matrilineal line following the death of Báthory (1586) when Sigismund III Vasa (son of Catherine Jagiellon) was elected in 1587. On 28 January 1588, Sigismund III confirmed the Third Statute of Lithuania which stated that the Polish–Lithuanian Commonwealth is a federation of two countries – Poland and Lithuania where both countries have equal rights within it and separated the powers of the ruler, the Seimas, the executive and the courts (this for the first time in European history ensured the rule of law in the state, but Lithuania's citizens, who were subjects to the Statute, were only nobles). During the Polish–Swedish War (1600–1611) Polish and Lithuanian forces achieved victory and restored status quo ante bellum, notably winning the decisive Battle of Kircholm in 1605, while during the Polish–Muscovite War (1605–1618) Polish and Lithuanian armies achieved territorial gains (e.g. restored the control of Smolensk, the capital of the Smolensk Voivodeship, in 1611) and for the first time fully captured Russia's capital Moscow in 1610. Sigismund III's son, Władysław IV Vasa, began ruling Lithuania in 1632 and achieved military success and popularity during the Smolensk War, but he renounced his claims to the Russian throne per the Treaty of Polyanovka in 1634 and failed at reclaiming the Swedish throne.

John II Casimir Vasa 's reign was initially marked with disastrous military loses as during the Deluge in the mid-17th century most of the territory of Lithuania was annexed by the Tsardom of Russia and even the Lithuania's capital Vilnius was captured for the first time by a foreign army and ravaged. In 1655, Lithuania unilaterally seceded from Poland, declared the Swedish King Charles X Gustav as the Grand Duke of Lithuania and fell under the protection of the Swedish Empire. However, by 1657 Lithuania was once again a part of the Commonwealth following the Lithuanian revolt against the Swedes. The Lithuania's capital Vilnius was liberated in 1661.

Throughout this Polish–Lithuanian Union period, the Grand Duchy of Lithuania remained a separate state and retained many rights in the federation (including separate name, territory, coat of arms, ministries, ruling system, laws, army, courts, treasury, and seal) until the Constitution of 3 May and Reciprocal Guarantee of Two Nations were passed in 1791.
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