In his life as a great revolutionary, Chairman Mao inherited, defended and developed Marxism-Leninism both in theory and in practice. Chairman Mao put forward the theory of the differentiation of the three worlds at a time when the two superpowers, the Soviet Union and the United States, became locked in a cutthroat struggle for world hegemony and were actively preparing for a new war. In appearance, this theory of Chairman Mao's seems to involve only relations between countries and between nations in the present-day world, but, in essence, it bears directly on the vital question of present-day class struggle on a world scale. Marxist-Leninists invariably adhere to the stand of the international proletariat, uphold the general interests of the revolutionary people of all countries in international class struggle and persist in the replacement of the capitalist system with the communist system as their maximum program.