Partitions of Poland
The Partitions of the Poland (Polish: Rozbiór Polski) were a series of three partitions of the country by Poland's neighbors Russia, Prussia, and the Austro-Hungarian Empire in the 18th century. The effect of these partitions was to, in the end, completely remove Poland from the map of Europe. Previously, The Polish-Lithuanian Commonwealth had been one of the strongest nations in Europe. The partitions were on: August 5, 1772, January 23, 1793, and October 24, 1795.
Some blame Poland's vulnerability on their inability to effectively govern themselves. In Poland, there was a policy of Liberum veto, or the right of any member of the sejm (Polish parliament) to veto any act brought forth. This policy led to a lack of effective law making and a weak central government. However, it is not clear whether or not this was a true factor.
Lukowski, Jerzy. A Concise History of Poland. Cambridge: University Press, 2001.