The Union of Lublin (1569) transformed the Lithuanian-Polish relations from those of two sovereign states sometimes sharing a single monarch to a single confederation of Poland-Lithuania. Lithuanians needed this union to secure their eastern boundaries from the strengthening Russians whereas a small, but more modern Polish kingdom sought for new lands (Lithuania agreed to transfer the entire Ukraine to Poland).
The Union bought time for both countries. Immediately after the unification Poles and Lithuanians conquered Livonia. German-ruled duchies of Courland and Prussia (a newly protestant state in place of the former Teutonic Order) became Polish-Lithuanian fiefs.
However, the pressure from other Eastern and Northern powers mounted and the turning point was the Deluge when Poland-Lithuania was invaded and partitioned by Russians and Swedes (1655 – 1660). Vilnius was desecrated and burned down by Russian Cossacks and a campaign of murders and rapes followed. This was the first sack of Vilnius since the Teutonic incursions.
Poland-Lithuania managed to liberate itself but after this blow never did it become a great power again. It continued to get weaker and weaker internally. After the extinction of Jagiellonian dynasty (1572), the monarchy became elective and every new monarch ceded more and more rights to the nobility. This way Poland-Lithuania acquired a unique political system of “Noble democracy” where most of the rights rested in approximately 10% of males known as the nobility (who participated in various parliaments or Seimas) with the rest of population having little or no power. Even the king had to follow the wishes of his noble “subjects”.
By the late 17th century the nobility enjoyed the right of “liberum veto” where any noble could stop any political decision he doesn’t agree with. The country became effectively paralyzed because consensus could never be reached with so many people enjoying the liberum veto right.
The state was further weakened by internal conflicts between different noble families, such as Radvila and Sapiega ones. These conflicts sometimes grew into civil wars for the delight of neighboring powers (e.g. in the year 1700).
Culturally Lithuania survived the reformation as a Catholic country (but minorities were respected). 46% of Grand Duchy's people were ethnic Lithuanians and 40% Belarusians but the nobility grew increasingly Polonized, adopting the Polish language which they regarded to be noble (whereas Lithuanian and Eastern Slavic languages were thought to be sufficient for peasants alone). The situation was remarkably different in Samogitia alone, where many nobles never ceased speaking Lithuanian natively.
With most Lithuanian speakers of Polish-Lithuanian Commonwealth illiterate, the center of ethnic Lithuanian culture moved to Lithuania Minor region in Prussia (east of Koenigsberg), where Lithuanians formed the majority of the population. In that area Reformation was successful and Lutheran faith became dominant. Lutherans promoted printing of religious books in local languages, including Lithuanian. Therefore the first Lithuanian books (e.g. “Katekizmas” by Martynas Mažvydas) were printed in Koenigsberg rather than Vilnius.
In the meantime, Polish-Lithuanian Commonwealth became weaker and weaker both internally and externally. In 1772 the three nearby great powers (Austria-Hungary, Prussia, and Russia) conspired to partition the country. Final attempts to change the situation, such as the adoption of a new Constitution (second in the world after the US one) abolishing the liberum veto, or the uprising by Tadeusz Kosciuszko, were too little and too late. After three partitions the Polish-Lithuanian Commonwealth was eradicated from the map. The ethnic Lithuanian lands were captured by Russia and Prussia in the third partition of 1795.