In 1492, the territory of Poland-Lithuania – not counting the fiefs of Mazovia, Moldavia, and Prussia – covered 1,115,000 km (431,000 sq mi), making it the largest territory in Europe; by 1793, it had fallen to 215,000 km (83,000 sq mi), the same size as Great Britain, and in 1795, it disappeared completely. The first 20th-century incarnation of Poland, the Second Polish Republic, occupied 389,720 km (150,470 sq mi), while, since 1945, a more westerly Poland covered 312,677 km (120,725 sq mi).

The Poles are the most numerous of the West Slavs and occupy what some believe to be the original homeland of the Slavic peoples. While other groups migrated, the Polanie remained in situ along the Vistula, from the river's sources to its estuary at the Baltic Sea. There is no other European nation centred to such an extent on one river. The establishment of a Polish state is often identified with the adoption of Christianity by Mieszko I in 966 CE (see Baptism of Poland), when the state covered territory similar to that of present-day Poland. In 1025 CE, Poland became a kingdom. In 1569, Poland cemented a long association with the Grand Duchy of Lithuania by signing the Union of Lublin, forming the Polish–Lithuanian Commonwealth. The Polish–Lithuanian Commonwealth was one of the largest and most populous countries in 16th- and 17th-century Europe.

The Polish–Lithuanian Commonwealth had many characteristics that made it unique among states of that era. The Commonwealth's political system, often called the Noble's Democracy or Golden Freedom, was characterized by the sovereign's power being reduced by laws and the legislature (Sejm), which was controlled by the nobility (szlachta). This system was a precursor to the modern concepts of broader democracy and constitutional monarchy. The two comprising states of the Commonwealth were formally equal, although in reality Poland was a dominant partner in the union. Its population was hallmarked by a high level of ethnic and confessional diversity, and the state was noted for having religious tolerance unusual for its age, although the degree of tolerance varied over time.

In the late 18th century, the Polish–Lithuanian Commonwealth began to collapse. Its neighbouring states were able to slowly dismember the Commonwealth. In 1795, Poland's territory was completely partitioned among the Kingdom of Prussia, the Russian Empire, and Austria. Poland regained its independence as the Second Polish Republic in 1918 after World War I, but lost it in World War II through occupation by Nazi Germany and the Soviet Union. Poland lost over six million citizens in World War II, emerging several years later as the socialist People's Republic of Poland within the Eastern Bloc, under strong Soviet influence.

During the Revolutions of 1989, communist rule was overthrown and Poland became what is constitutionally known as the "Third Polish Republic." Poland is a unitary state made up of sixteen voivodeships (Polish: województwo). Poland is a member of the European Union, NATO, and the Organisation for Economic Co-operation and Development (OECD).

Poland currently has a population of over 38 million people, which makes it the

Read more on Wikipedia

In the period following the emergence of Poland in the 10th century, the Polish nation was led by a series of rulers of the Piast dynasty, who converted the Poles to Christianity, created a sizeable Central European state, and integrated Poland into European culture. Formidable foreign enemies and internal fragmentation eroded this initial structure in the 13th century, but consolidation in the 14th century laid the base for the Polish Kingdom.

Beginning with the Lithuanian Grand Duke Jogaila, the Jagiellon dynasty (1385–1569) ruled the Polish–Lithuanian union. The Lublin Union of 1569 established the Polish–Lithuanian Commonwealth as an influential player in European politics and a vital cultural entity.

Territorial changes before and during the Kingdom of Poland (1025–1385), ending with the Union of Krewo.
Mieszko I of Poland was the first historical ruler of the first independent Polish state ever recorded- Duchy of Poland. He was responsible for the introduction and subsequent spread of Christianity in Poland. During his long reign most of the territories inhabited by Polish tribes were added to his territory into a single Polish state. By c. 967 he included Pomerania up to the Oder estuary and Wolin island in the west. The 10th-century western border of Poland in the Pomeranian section was probably based on the Rędowa River, i.e. further west than today. In 981 he lost the Czerwień Cities in the south-east, tribal territory of the Lendians, to the Kyivan Rus'. The last of his conquests were Silesia and south-western Lesser Poland that were incorporated some time before 990.
During the reign of Bolesław the Brave, relations between Poland and the Holy Roman Empire deteriorated, resulting in a series of wars (1002–1005, 1007–1013, 1015–1018). Poland took control of the Lusatia region in the south-west, and briefly Miśnia. From 1003 to 1004 Bolesław intervened militarily in Czech dynastic conflicts. After his forces were removed from Bohemia in 1018, Bolesław retained Moravia. In 1013, the marriage between Bolesław's son Mieszko and Richeza of Lotharingia, the niece of Emperor Otto III and future mother of Casimir I the Restorer, took place. The conflicts with Germany ended in 1018 with the Peace of Bautzen accord, on favorable terms for Bolesław, retaining control of Lusatia. In the context of the 1018 Kiev expedition, Bolesław retook the Czerwień Cities. In 1019, Poland lost Brześć to the Kyivan Rus', but retook it the following year. In 1025, shortly before his death, Bolesław I the Brave finally succeeded in obtaining the papal permission to crown himself, and became the first king of Poland.
The first Piast monarchy collapsed after the death of Bolesław's son – king Mieszko II in 1034. Deprived of a government, Poland was ravaged by an anti-feudal and pagan rebellion, and in 1039 by the forces of King Bretislav of Bohemia. The country suffered territorial losses, and the functioning of the Gniezno archdiocese was disrupted.

According to various sources, either in 1042 or 1044 Poland lost Brześć to the Kyivan Rus'.

After returning from exile in 1039, Duke Casimir I (1016–1058), properly known as the Restorer, rebuilt the Polish monarchy and the country's territorial integrity through several military campaigns: in 1047, Masovia was taken back from Miecław, and in 1050 Silesia from the Czechs. Casimir was aided by the recent adversaries of Poland, the Holy Roman Empire and Kievan Rus, both of whom disliked the chaos in Poland. Casimir's son Bolesław II the Generous managed to restore most of the cou…

Read more on Wikipedia