Poland's earliest form as a state was around 966, under the Piast dynasty. My various internet sources point to the conversion of Miezko I to Christianity as the founding moment of Poland, although it was his son Boleslaw who was the first officially crowned King of Poland, approved by the Pope. Fragmentation stemming from Boleslaw III in the 1100's eventually led to the Union of Krewa in 1385, between Poland and Lithuania. Although Poland and Lithuania remained separate states ruled by separate kings, essentially the kings were Lithuanian. The first Lithuanian King of Poland was Ladislaw II, Grand Duke of Lithuania who married the rightful Queen of Poland and converted to Christianity. Poland became a constitutional monarchy in which the succession of throne must be approved by consensus of Nobility, Lithuania became Christian.
In 1569, yet another union with Lithuania led to the the Polish-Lithuanian Commonwealth. Poland became an elected monarchy under which the nobility enjoyed Golden Freedom. The nobility legislated laws and elected the monarch as well as enjoying religious tolerance unheard of elsewhere in that era. At this point Poland was home to the largest population of European Jews (and had been for almost 2 centuries). Unfortunately the lesser nobility were not afforded the same rights as the land-holding nobility, which combined with a series of wars (with Russia, Ukraine, Sweden) in the 1600s weakened the Polish-Lithuanian commonwealth and marked the decline of the Polish Golden Age.
Although Poland fought to maintain independance in the late 1700s the country was partitioned by Russia, Prussia and Austria. In 1807, Napoleon defeated Prussia and set up a Polish state, the Duchy of Warsaw, under French rule, adding to it's mass two years later when he defeated Austria. After Napolean was defeated the Duchy of Warsaw was renamed the Kingdom of Poland under the Russian Czar before finally being incorporated into Russia.
After WWI, Poland declared independance in 1918, which was followed rather quickly by the Polish victory in the Polish-Soviet War in 1919. Joseph Pilsudski led Poland in stability until the late 1930s, when the Great Depression and WWII took their toll. In 1939 Poland entered full military alliance with Britain and France. Later that year in alliance with Nazi Germany, the Soviet Union invaded Poland. In 1941 Germany invaded the Soviet Union and took complete control of Poland, although a Polish government in exhile remained which controlled Polish armed forces operating outside of Poland alongside the Red Army of the Soviet Union. During the war the Polish people continually to resisted the Nazi government. In 1944 the Soviet army entered Poland and defeated the Germans.
In 1945 an independant Polish state was formed. The Communist Party controlled the elections of 1947, however, and so a communist regime was established. In the early 1980s, a Solidarity movement began which was the first non-communist trade union under a communist regime. The government attempted to destroy the union by declaring martial law, but ultimately had to negotiate with the union. These negotiations eventually led to semi-free elections in 1989 and ultimately the end of communism in Poland. Poland's first free parliamentary elections were held in 1991 and in 1993 the remnants of the Soviet army finally left Poland.