It broke up because of the partitions. Every Polish school child knows that.
I'm not a Polish school child, I know better.
Why did the Partitions happen ?
The Commonwealth was facing many internal problems and was vulnerable to foreign influences. Chmielnicki's Uprising. Swedish invasion in 1655.
The war between the King and the nobles in 1715. The Silent Sejm. Liberum Veto. The Commonwealth was rotting inside.
Next question: Why was it rotting inside ? Why has the Chmielnicki's Uprising happened ? Why was Sweden - a much smaller country - able to invade ?
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PiS has appointed Stanisław Piotrowicz as a head of Sejm's Justice and Human Rights Commision. In 2001, he closed the trial of a priest from Tylawa, who was accused of child molesting. He explained the priest's behaviour as "fatherly tenderness", and practicing "energy medicine". Staying overnight was "an attraction", bathing children was necessary because they were dirty.
3 years later, the priest was sentenced for 2 years in prison for touching little girls, putting fingers inside them, etc.
Piotrowicz was a member of PZPR, too.