In the Eastern European countries, each Communist party prevented opponents from organizing themselves politically, but opposition manifested itself against the "model" that each single Communist party had to follow. Despite Russian power, however, opposition to the Soviet model assumed a clear ideological character after the signing of the Eastern European Mutual Assistance Treaty on May 14, 1955. An exact picture of the religious opposition in all of its manifestations, of the different churches, especially the Catholic Church, in the countries of Eastern Europe is still difficult to apprehend in all its details. Marxism is an international doctrine, but in Eastern Europe Soviet Marxism imposed directives dictated by Russian military might. In the succeeding months the reformers lost all illusion that the Communist parties of Eastern Europe could conduct any substantial revision of the socialist system. In Czechoslovakia criticism of the socialist system began with the criticisms of Communist party economists in the early 1960s.