For the historian, the parallels between the Commonwealth of Poland-Lithuania and the United Kingdom of Great Britain and Ireland in the early eighteenth century are striking, even if the subsequent fates of the two states mean that direct comparisons have rarely been undertaken. Both were union states that had transcended the loose personal or dynastic unions that characterised the political structures of much of late medieval and early modern Europe by implementing the only two full parliamentary unions seen in Europe before 1789: between Poland and Lithuania in 1569, and between England and Scotland in 1707. Like Scotland after 1707, Lithuania kept its own legal system and its own laws after 1569. In both cases, the unions were part of a wider union state in which other parts had a different relationship to the centre: in the United Kingdom, Wales had been incorporated into England in the 1530s, while Ireland had its own parliament until 1801; in Poland-Lithuania, Royal Prussia was included in the 1569 parliamentary union, but had wide privileges, including the retention of its own law codes and urban representation in its Landtag, while the Ukrainian palatinates incorporated into Poland from Lithuania in 1569 kept Lithuanian law and their own chancery records, the Metryka Ruska.

The similarities were not just structural. Both systems struggled with the problems of religious pluralism and both drew heavily in their political thought and artistic culture on the heritage of the Roman Republic with its defence of liberty against dictators or kings who sought to undermine it: Joseph Addison's great tragedy Cato, which opened in April 1713 to wild acclaim from Whigs and Tories alike, was not translated into Polish until 1809, but its sentiments suffused Polish political culture. When Addison's Cato states that ‘A day, an hour of virtuous liberty/ Is worth a whole eternity in bondage’, he is expressing a common Polish sentiment: malo periculosam libertatem quam quietam servitutem (better in perilous liberty than in quiet servitude), a phrase echoed by Rousseau and made famous by Thomas Jefferson as the Americans boldly constructed their own version of the republican vision.