Latvia has been inhabited since the third millennium BC, and became an early centre of the European trade. The amber found in Latvia was as valuable as gold, being the material of precious jewels in the ancient Greece and the Roman Empire. The territory of Latvia has always been the target of many nations, because of it's strategic location. In the 12th century German traders and preachers established settlements in Latvia. The largest of them, Riga, founded in 1201 became part of the Hanseatic League. In the 16th century Latvia became part of the Polish-Lithuanian Kingdom, but in the 17th century it came under Swedish rule for a century. In the 18th century Russia was victorious in the northern wars, bringing Latvia into Imperial Russia. In the 19th century Riga became a large industrial centre and one of the largest cities in the Russian Empire. Russian influence weakened after the Russian Revolution, and Latvia regained independence. The country had a brief period of democracy and economic recovery, but a coup in 1934 established a nationalist dictatorship. The new regime tried to keep it's neutrality and independency in the early years of the Second World War, but it's fate was decided by the Molotov-Ribbentrop pact. Soviet troops re-occupied Latvia, and remained there until 1994. Latvia became independent after the collapse of the Soviet Union in 1991. The democratic government returned to the old currency, denationalized the state property, liberated the market and successfully reoriented the country to the Western states. Latvia has been a member of the European Union since May 1, 2004, and member of the NATO since April 2, 2004.