Poland gained its independence in 1918 only to be overrun by Germany and the Soviet Union in World War II. It became a Soviet satellite country following the war, but one that was comparatively tolerant and progressive. Labor turmoil in 1980 led to the formation of an independent trade union "Solidarity" that over time became a political force and by 1990 had swept parliamentary elections and the presidency. Complete freedom came with the implosion of the USSR in 1991. A "shock therapy" program during the early 1990s enabled the country to transform its economy into one of the most robust in Central Europe, boosting hopes for early acceptance to the EU. Poland joined the NATO alliance in 1999.