In August 1968, the Polish People's Army took part in the Warsaw Pact invasion of Czechoslovakia. Some Polish intellectuals protested, and Ryszard Siwiec burned himself alive during celebrations of an official holiday. The Polish participation in the crushing of the Czech liberalization movement (the crowning achievement of Marxist revisionism, according to David Ost) further alienated Gomułka from his former liberal supporters. But within the party, the opposition to Gomułka faded and the 5th Congress of the PZPR reconfirmed his rule in November. Brezhnev, who attended the gathering, used the occasion to expound his Brezhnev Doctrine, a self-granted Soviet right to forcefully intervene if an allied state strays too far from the "fraternal course".