The Partitions of Poland (more correctly the partitions of Poland-Lithuania) happened in the 18th century and ended the existence of a Polish sovereign state. They involved Prussia, Russia and Austria dividing up the Polish lands between themselves. The three partitions occurred: The term "Fourth Partition" may refer to one of a number of subsequent divisions of the Polish lands, specifically:

Table of contents
1 Preludium
2 First Partition
3 Second Partition
4 Third Partition
5 Consequences

Preludium

In traditional history one can find the claim that the regional powers partitioned Poland-Lithuania because of the degeneration of the state and because of the inability of the Poles to rule themselves. Apart from the semi-racist implications of that view, one can discount this suggestion, since the darkest period of Polish history and the nadir in the degeneration of the state occurred in the first half of the 18th century, whereas the partitions happened when Poland had started (slowly) to recover - in fact one can see the last two partitions as a direct answer to reforms in Poland-Lithuania.

In other words, the partitions did not happen because Poland was a degenerate, weak and backward country; rather, Poland suffered partitioning because it was weak, backward, and tried to reform itself.

One could characterise Poland-Lithuania before the partitions as already not a completely sovereign state: in modern terms it comprised a Russian satellite state, with Russian Tsars effectively choosing the Polish kings. 

The neighbours of Poland, Prussia, Austria and Russia, signed a secret agreement in order to maintain the status quo: specifically, to ensure that Polish laws would not change. Their alliance later became known as the "Alliance of the Three Black Eagles", because all three states used a black eagle as a state symbol. (Compare the white eagle as a symbol of Poland.)

The Poles tried to expel foreign forces in an uprising (Bar confederation, 1768-1772), but the irregular and poorly commanded forces had no chance in face of the regular Russian army and suffered crushing defeat.

First Partition

Why it happened, reforms in Sejm

Second Partition

Constitution of third may, war of constitution,, targowica

Third Partition

Kosciuszko uprising

Consequences

trauma for Poles, neverending source of troubles for Europe (alliance with Napoleon, uprisings: 1830-1,1846,1848,1863,1905...)

see also: History of Poland