Poland is a country in Central Europe, bordered by Germany to the west; the Czech Republic and Slovakia to the south; Ukraine, Belarus and Lithuania to the east; and the Baltic Sea and Russia's Kaliningrad Oblast to the north. It is an ancient nation that was conceived near the middle of the 10th century. Its golden age occurred in the 16th century when it united with the Grand Duchy of Lithuania to form the Polish–Lithuanian Commonwealth. During the following century, the strengthening of the gentry and internal disorders weakened the nation. In a series of agreements in the late 18th century, Russia, Prussia and Austria partitioned Poland amongst themselves. Poland regained its independence as the Second Polish Republic in 1918, after World War I, but lost it again in World War II, then occupied 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. Labor turmoil in 1980 led to the formation of the independent trade union "Solidarity" that over time became a political force which by 1990 had swept parliamentary elections and the presidency. A shock therapy program during the early 1990s enabled the country to transform its economy into one of the most robust in Central Europe. With its transformation to a democratic, market-oriented country largely completed, Poland is an increasingly active member of NATO and the European Union.