His educational opportunities having been limited by his bourgeois background (when freedoms were limited by the Czechoslovak Socialist Republic), Havel first rose to prominence as a playwright. In works such as "The Garden Party" and "The Memorandum", Havel used an absurdist style to criticize communism. After participating in the Prague Spring and being blacklisted after the Warsaw Pact invasion of Czechoslovakia, he became more politically active and helped found several dissident initiatives, including Charter 77 and the Committee for the Defense of the Unjustly Prosecuted. His political activities brought him under the surveillance of the secret police and he spent multiple periods imprisoned, the longest being nearly four years, between 1979 and 1983.