In "The Future of Mankind," Russell foresees one of three possibilities for the future of mankind:
I. The end of human life, perhaps of all life on our planet.
II. A reversion to barbarism after a catastrophic diminution of the population of the globe.
III. A unification of the world under a single government, possessing a monopoly of all the major weapons of war.
Russell supposes that the end of all life could occur but it would not occur until the means of technology reached a point where/when an atomic war, and its radioactive aftermath, would be able to annihilate all life on the planet. Russell does not suppose that I, II, or III is more likely than the other. He simply speculates how and when each scenario might occur.
Russell supposes that a return to barbarism could occur, also after a world/atomic war. He imagines that cities, libraries, and educated people would all be destroyed and that prophets would blame science itself. Thus, states would break up and the world would become more primitive. The only consolation in this scenario is that a return to civilized society would remain possible.
Russell suppose that a unification of a world government could result from an alliance between Russia and the United States (this was during the Cold War) or if either Russia or the US would win the next world war. Russell's most hopeful outcome would be the establishment of a united world government so powerful that Russia would feel compelled to go along with it. However, Russell fears that force would be necessary. In order to prevent a catastrophic atomic world war, Russell argues in favor of a united world government, one which is in charge of the world's nuclear arsenal and any other technological means of mass destruction. This, he feels, would prevent scenario I or II from occurring.
In this essay, Russell prefers an alliance between Russia and the US but if it came to war, he prefers a United States' victory to a Russian victory because the United States' culture is more open to free thinking.