Incumbent governments have transferred power smoothly and constitutionally in every instance to their successors . Poland was ruled by Soviet - backed government after the Second World War . Solidarity movement was a key factor in the collapse of communist regime . Centrally - planned economic system replaced by free market economy . Poland joined the European Union in 2004 .
Poland was ruled by Soviet - backed government after the Second World War . Solidarity movement was a key factor in the collapse of communist regime . Centrally - planned economic system replaced by free market economy . Poland joined the European Union in 2004 .