Poland is valuable, and coveted, and it was invaded or annexed numerous times throughout its history, most notably by the Tatars in the 13th century (the Mongol Invasion) and the Swedes in the 17th century (the Swedish Deluge).
By the 18th century, its government was ahead of its time and Polish ideals were used as a model for the United States government, but what did the country in was the Liberum Veto that allowed any member of the legislature to stop the session and nullify passed legislation by shouting Nie pozwalam! (“I do not allow!”). Paul Krugman wrote about it in http://krugman.blogs.nytimes.com/2010/02/05/the-senate-becomes-a-polish-joke.
This stubborn inability of the rulers of Poland to get things done and the weakness that resulted allowed the Kingdom of Prussia, the Russian Empire, and Habsburg Austria to take over pieces of what had been the Polish–Lithuanian Commonwealth in 1772, 1793, and 1795.
Even when the country of Poland did not officially exist, the dream of a restored Poland persisted. The optimism of the people is reflected in the national anthem of Poland Mazurek Dąbrowskiego (Dąbrowski’s Mazurka), which has been translated as “Poland Is Not Yet Lost” or “Poland Has Not Perished Yet.”
James Michener wrote about the government and partitions informally in his historical novel, Poland, but for more specifics, see http://en.wikipedia.org/wiki/Partitions_of_Poland.