Complex sentence: In effect, this meant the Soviet Union would agree to a test ban in the atmosphere, outer space, and under water environments; a position the Western powers had long favored as an alternative to a more comprehensive (underground environment) ban.
Split into simpler sentences: In effect, this meant the Soviet Union would agree to a test ban in the atmosphere, outer space, and under water environments. This was the position that the Western Powers had long favored as an alternative to a more comprehensive (underground environment) ban.