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 whose history as a state began 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. It regained independence as the Second Polish Republic in the aftermath of World War I only to lose it again when it was occupied by Nazi Germany and the Soviet Union in World War II. The nation lost over six million citizens in the war, following which it emerged as the communist Polish People's Republic under strong Soviet influence within the Eastern Bloc. A westward border shift followed by forced population transfers after the war turned a once multiethnic country into a mostly homogeneous nation state. Labor turmoil in 1980 led to the formation of the independent trade union called Solidarity (Solidarność) 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 completed, Poland joined NATO in 1999 and the European Union in 2004, but has experienced a constitutional crisis and democratic backsliding under the rule of the populist Law and Justice party since 2015.