During the reign of John II Casimir Vasa (r. 1648–1668), the third and last king of his dynasty, the nobles' democracy fell into decline as a result of foreign invasions and domestic disorder. These calamities multiplied rather suddenly and marked the end of the Polish Golden Age. Their effect was to render the once powerful Commonwealth increasingly vulnerable to foreign intervention.

The Cossack Khmelnytsky Uprising of 1648–1657 engulfed the south-eastern regions of the Polish crown; its long-term effects were disastrous for the Commonwealth. The first liberum veto (a parliamentary device that allowed any member of the Sejm to dissolve a current session immediately) was exercised by a deputy in 1652. This practice would eventually weaken Poland's central government critically. In the Treaty of Pereyaslav (1654), the Ukrainian rebels declared themselves subjects of the Tsardom of Russia.

The Second Northern War raged through the core Polish lands in 1655–1660; it included a brutal and devastating invasion of Poland referred to as the Swedish Deluge. During the wars the Commonwealth lost approximately one third of its population as well as its status as a great power due to invasions by Sweden and Russia. According to Professor Andrzej Rottermund, manager of the Royal Castle in Warsaw, the destruction of Poland in the Deluge was more extensive than the destruction of the country in World War II. Rottermund claims that Swedish invaders robbed the Commonwealth of its most important riches, and most of the stolen items never returned to Poland. Warsaw, the capital of the Polish–Lithuanian Commonwealth, was destroyed by the Swedes, and out of a pre-war population of 20,000, only 2,000 remained in the city after the war. The war ended in 1660 with the Treaty of Oliva, which resulted in the loss of some of Poland's northern possessions.

The large-scale slave raids of the Crimean Tatars also had highly deleterious effects on the Polish economy. Merkuriusz Polski, the first Polish newspaper, was published in 1661.