In 1945-6 almost the entire German minority of pre-War Czechoslovakia, about 3 million people, were expelled to Germany and Austria. During this time, hundreds of thousand of Germans were held in prisons, detention camps, and used in forced labour. In the summer of 1945, there were several massacres with at least 20,000 deaths. Only 250,000 Germans who had been active in the resistance against the Nazis or were necessary for the economy were allowed to stay, though many of them emigrated later. Czechoslovakia uneasily tried to play "bridge" between the West and East; however the strengthening Communist Party of Czechoslovakia took over in February 1948, bringing the country within the Soviet sphere of influence. In August 1968, an invasion by Warsaw Pact troops ended the efforts of the country's leaders to liberalize party rule and create "socialism with a human face" during the Prague Spring. In November 1989, Czechoslovakia regained its political independence through a peaceful "Velvet Revolution". On January 1, 1993, the country peacefully split in two, creating the independent Czech and Slovak republics. The Czech Republic joined NATO on March 12, 1999 and the European Union on May 1, 2004. From 1991 the Czech Republic, then Czechoslovakia, has been a member of the Visegrad Group.