Beginning in the 1960s, Havel mostly wrote about the politics of Czechoslovakia. After the Prague Spring, he became more and more active against the government. In 1977, he became famous internationally for his work on the human rights manifesto, "Charter 77". He became known as a leader of the opposition in Czechoslovakia. He was also sent to prison for these activities. The 1989, Havel became president during the "Velvet Revolution". As president, he led Czechoslovakia and later the Czech Republic to an open democracy with several political parties. His country changed greatly during the thirteen years he was president. The Czech Republic separated from Slovakia, even though Havel was against separation. The Czech Republic also joined NATO and started negotiating membership in the European Union. The country became a member of the EU in 2004. He was one of the first people to sign the Prague Declaration on European Conscience and Communism.